John,

You forgot to give us your CYA and total Chlorine readings...just kidding, man.

You have the same problem, I have. You filter is clogged.

Not sure why because you didn't give enough specifics, but it could be a few things.

I plugg my skimmer in the winter- intentionally. Is your skimmer plug out?

Does your skimmer line have shut-off valve? Is it open?

My guess is that neither of these are your problem because you have 20 lbs at the filter, so it must be getting water from the bottom drain or your spa, just not the skimmer. But if this were the case and the filter was getting adequate supply (no cavitation at the pump-right?) then were is that water going???? You'd have good flow at the return...but alas you don't. Therefore your filter is plugged up.

Could be one big DE cake somewhere. A dirty filter nest or my last though, a return valve that might not be open.

Just some food for thought...

Rod