If it's for both guitars, I guess it's probably not the guitars themselves.
Having said that, old strings can cause loss in sustain or if they've both got damp or something. Or muck on the bridge, or if you've changed gauge of strings to lighter ones.
Do they both play with good sustain unplugged ? ie, same as before ?
Assuming the guitars are ok, and your effects board hasn't changed to cause the poor sustain ..
Try swapping out the leads. Sometimes leads do weird things on their way out as the resistance or conections become unreliable. A muting /muffling effect isn't uncommon in my experience.
Don't forget this could also be the patch leads between effects
Check the voltage to your effects board. If it's lower than it should be, this can cause odd thigns to happen too. This is more likely if you use a universal power supply capable of several voltage ranges. Is the issue still there when not connecting through your pedalboard?
Amp. I don't know what kind of amp you have but I have noticed a particular kind of fault where you play a chord or note, it's loud (as usual) for maybe a quarter of a second, then it dies prematurely. Next note played seems to 'peak' through the issue but dies again soo after. I've found this ona few amps as jam nights and one of my own. I don't know the cure unfortunaely- probably something for a technician to look into. I guess the test would be to try someone else's amp, ideally one similar to your own.
Have you changed your technique lately ? Maybe it's finger-error ? This seems unlikely but I'm just trying to think of everything
If you want more sustain, a compressor will do that. They smooth the sound out very well ! But I guess that doens't help you find the real cause.