Without further information (i.e. stderr.2 logs) it's hard to say for sure what the problem is. Most likely although your fft plot "looks" good you may not be accurately tuned. Typically you'll need to adjust ppm (-q parameter) to bring the tuning into alignment enough that the p25_demod locks on. When that happens you should see the TSBK counter start to increment.

If you're using the "boatbod" fork you can use the mixer plot (key #5) to dial in the ppm and fine tuning more accurately than is possible with the fft plot alone. Furthermore, you can also use a nac value of 0 if you don't know or cannot verify the nac of the system you are trying to tune.