BMODES results interpretation

Hi again,
I obtain a strange output from an otherwise perfectly reasonable BMODES run. Modes n. 11 and 18 are completely zero, although with a normal frequency.
I do not care particularly about so high-numbered modes, I just wanted to check if this is normal, and/or could point to some input error. The other modes seem normal.

Here an excerpt of the output:

Results generated by BModes (v1.03.01, 25-Sept-2007, compiled using double precision) on 30-Nov-2007 at 11:56:00.
Modi propri della torre           
=================================================================================

            tower frequencies & mode shapes
            --- only first  20 modes printed


 -------- Mode No.   1  (freq = 0.18701E+01 Hz)

span_loc	s-s disp	s-s slope	f-a disp	f-a slope	twist

 0.0000	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.0017	 0.000000	-0.000002	 0.000000	 0.000000	 0.000004
 0.0019	 0.000000	-0.000003	 0.000000	 0.000000	 0.000005
 0.0898	-0.000016	 0.000318	 0.000000	 0.000000	 0.003209
 0.1780	 0.000167	 0.004067	 0.000000	 0.000000	 0.006742
 0.2659	 0.000674	 0.006791	 0.000000	 0.000000	 0.010985
 0.2661	 0.000675	 0.006791	 0.000000	 0.000000	 0.010986
 0.2695	 0.000698	 0.006793	 0.000000	 0.000000	 0.011003
 0.2697	 0.000699	 0.006794	 0.000000	 0.000000	 0.011004
 0.3595	 0.001330	 0.007476	 0.000000	 0.000000	 0.015866
 0.4495	 0.002248	 0.014876	 0.000000	 0.000000	 0.022025
 0.5208	 0.003902	 0.033422	 0.000000	 0.000000	 0.027409
 0.5919	 0.007353	 0.065054	 0.000000	 0.000000	 0.033605
 0.5921	 0.007365	 0.065072	 0.000000	 0.000000	 0.033608
 0.5956	 0.007592	 0.065259	 0.000000	 0.000000	 0.033641
 0.5957	 0.007605	 0.065278	 0.000000	 0.000000	 0.033644
 0.6858	 0.016295	 0.128890	 0.000000	 0.000000	 0.043494
 0.6859	 0.016320	 0.129055	 0.000000	 0.000000	 0.043517
 0.7758	 0.032124	 0.223495	 0.000000	 0.000000	 0.056274
 0.8656	 0.057216	 0.331158	 0.000000	 0.000000	 0.071655
 0.9592	 0.092114	 0.402922	 0.000000	 0.000000	 0.089122
 0.9594	 0.092190	 0.402935	 0.000000	 0.000000	 0.089128
 0.9611	 0.092871	 0.402940	 0.000000	 0.000000	 0.089131
 1.0000	 0.108559	 0.402976	 0.000000	 0.000000	 0.089168

[....]


 -------- Mode No.  10  (freq = 0.43435E+02 Hz)

span_loc	s-s disp	s-s slope	f-a disp	f-a slope	twist

 0.0000	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.0017	 0.000000	-0.000001	 0.000000	 0.000000	 0.000050
 0.0019	 0.000000	-0.000001	 0.000000	 0.000000	 0.000061
 0.0898	-0.000029	-0.000555	 0.000000	 0.000000	 0.041257
 0.1780	-0.000083	-0.000597	 0.000000	 0.000000	 0.085269
 0.2659	-0.000123	-0.000292	 0.000000	 0.000000	 0.129439
 0.2661	-0.000123	-0.000292	 0.000000	 0.000000	 0.129456
 0.2695	-0.000124	-0.000291	 0.000000	 0.000000	 0.129611
 0.2697	-0.000124	-0.000290	 0.000000	 0.000000	 0.129626
 0.3595	-0.000131	 0.000125	 0.000000	 0.000000	 0.167627
 0.4495	-0.000101	 0.000542	 0.000000	 0.000000	 0.198409
 0.5208	-0.000050	 0.000862	 0.000000	 0.000000	 0.214677
 0.5919	 0.000019	 0.001046	 0.000000	 0.000000	 0.221482
 0.5921	 0.000019	 0.001046	 0.000000	 0.000000	 0.221483
 0.5956	 0.000023	 0.001046	 0.000000	 0.000000	 0.221472
 0.5957	 0.000023	 0.001047	 0.000000	 0.000000	 0.221470
 0.6858	 0.000113	 0.000888	 0.000000	 0.000000	 0.203559
 0.6859	 0.000113	 0.000887	 0.000000	 0.000000	 0.203492
 0.7758	 0.000170	 0.000401	 0.000000	 0.000000	 0.156993
 0.8656	 0.000192	 0.000150	 0.000000	 0.000000	 0.084573
 0.9592	 0.000206	 0.000247	 0.000000	 0.000000	-0.008156
 0.9594	 0.000207	 0.000247	 0.000000	 0.000000	-0.008188
 0.9611	 0.000207	 0.000247	 0.000000	 0.000000	-0.008202
 1.0000	 0.000217	 0.000249	 0.000000	 0.000000	-0.008367


 -------- Mode No.  11  (freq = 0.57267E+02 Hz)

span_loc	s-s disp	s-s slope	f-a disp	f-a slope	twist

 0.0000	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.0017	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.0019	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.0898	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.1780	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.2659	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.2661	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.2695	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.2697	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.3595	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.4495	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.5208	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.5919	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.5921	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.5956	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.5957	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.6858	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.6859	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.7758	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.8656	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.9592	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.9594	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.9611	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 1.0000	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000


 -------- Mode No.  12  (freq = 0.78742E+02 Hz)

span_loc	s-s disp	s-s slope	f-a disp	f-a slope	twist

 0.0000	 0.000000	 0.000000	 0.000000	 0.000000	 0.000000
 0.0017	 0.000000	 0.000341	 0.000000	 0.000000	-0.000005
 0.0019	 0.000000	 0.000411	 0.000000	 0.000000	-0.000006
 0.0898	 0.008737	 0.158468	 0.000000	 0.000000	-0.003822
 0.1780	 0.021406	 0.095603	 0.000000	 0.000000	-0.007204

Thanks a lot for your time and hints.
Best regards and happy weekend from Italy
Claudio