Back to basics:
#1 if we are talking MTH engines with DCS, then the TIU channels ONLY send the watchdog signal on power up of that channel.
#2 Hopefully, with multiple channels being used for block sections- along with rule #1 you are aware that the TIU logic needs power. The one way is assuming- and right there is how you mess up, that Fixed channel 1 has constant power, but if you are using a ZW-L variable transformer- then most likely you should have aux power into the TIU so the TIU logic remains powered regardless of what input you might be powering.
#3 Light signals and other accessories attached to the track may attenuate or interfere with the DCS signal. It's recommended in many cases to install chokes (AKA inductors) in series with those accessories to prevent them from attenuating the DCS signal.
#4 the number of cars and engines can ALSO affect signal.
Not to mention, the other obvious track related things:
Are both outside rails bonded together periodically or did you just power center and one outer rail?
Did you have a blackened center rail?
With multiple adjacent tracks, TMCC/Legacy signally can be affected.
Has your power strip changed and might you have lost ground connection required for TMCC signaling?
