Hi Tim,
There are many possibilities.? Is it low on all bands or only one?? It could be that the TR MOSFET (Q508) is not working.? Are you getting good sideband separation using the audio sweeps?? Are the values of the RF sweeps at the FT8 frequency above -10?
The most common problem is a poor solder joint on one of the toroids.
73
Evan
AC9TU
