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.