converting SU file to ASCII format in seismic unix using OpenSeaSeis
The number of samples in the input binary header from the SEGY file does not match the NSAMP trace header.  I would first confirm: 


What # of samples is in the binary SEG-Y header. 
The number of samples you expect in each trace
Dump the su trace header for     number of samples. You can use
SUGETHW and the keyword for # of samples is "ns".


If the error is in an individual trace header, you can overwrite that with SUSETHW.