There's 3 things it could be, easiest first:

1. [Radios] section in aircraft.cfg Nav1 is disabled:

[Radios]
// Radio Type = availiable, standby frequency, has glide slope
Audio.1= 1
Com.1= 1, 0
Com.2 = 1, 0
Nav.1= 0, 0, 1
Nav.2= 1, 0, 1
Adf.1= 1
Adf.2= 1
Transponder.1 = 1
Marker.1= 1

2. The CAB file 'Captain_Sim.p721.L15.cab' was changed in v2.7

3. The interior file 'cs727_200_interior.mdl' was changed in v2.7

It appears you mainly fly the -200. Have you confirmed that the -100 doesn't work either?