It's your clock likely. Check your clock vs time.is
Needs to be almost perfectly sync on cpu
Discussion
I'll have to figure out how to set the clock on it.
It's the clock on your computer running ft8. It needs to be with I think 300ms of the signal senders clock.
Everytime my ft8 doesn't decode, that's what the culprit is. My clock has slightly drifted from global sync.
It doesn't have to be perfect. When I am operating FT8 in my truck (parked), I am often up to 3 seconds off and still get some decodes.