loss of pressure !?!?!

I bought a house with an AGP 24' round. It has a hayward filter which the manual says holds 50 lbs. My problem is this: I vacuum for 5 minutes and the pressure drops to nothing and the vac plate comes off the skimmer.
I then backwash the filter and the pressure returns, but the same thing happens all over again.
If I leave the filter on overnight, I find that in the morning, the pressure is gone then too. Help!?!?
My friend says I need a bigger filter. But it seems to be the one that came with the pool, so it should work, shouldn't it? Anyway. . .Help!

Unfortunately just because it came with the pool doesn't mean it is the right size. If I'm understanding you correctly your sand filter only holds 50lbs of sand. That is pretty small for a 24 ft pool. What is your pump rating?

Are you seeing air coming into the pool from the filter return when you lose your pressure?

Not sure of pump rating. . .But not seeing air bubbles that you describe. Return puts out quite a lot of volume at good pressure, but inexplicably, (or so it seems) the pressure drops to nealry nil and vacuum plate drifts off its place on the skimmer. Then I need to backwash again. . .and on and on like that. . .

Can you post a picture of your pump and filter? Does your pump not have the hp posted on it?

There are a variety of reasons you could be losing pressure. You may have a partial clog somewhere, your filter may be too small, your pump may be too weak (normally pumps are stronger than necessary so I'm not banking on that one), you could be sucking air from somewhere, or you might just have your pump filter basket turned so it is blocking flow into the filter (i did that this spring and had a resulting loss of pressure).

Those are the easy ones I can think of. More information may help us give you suggestions.

As of yesterday, it's now a non-issue! My pump died. I figure that must have been the reason for the loss of pressure all along. I replaced the sand filter with a cartridge filter, and got a new pump and everything seems to be OK. Thanks for your replies!