You could have plenty of signal and still have a snowy picture.
Cable amps are built non-identical. Since no ckt has a flat response,
if they were the same you would have some channels with lots of signal
and some with little. They make them different so that it balances
out. If you put an amp in line and channels are still snowy or you get
overload, this may be the problem. - they don't have them random
enough.

Go direct to the source first. If it is good on ALL channels, then
your wiring is the problem.

Cable has a tendency to "shrink" the center conductor, and you have to
redo the cable ends. Check to make sure the center conductor is
protruding far enough.