Newby with vac problem

Hello everyone,
I have had a vinyl pool for 15 years in my current location with no real problems until the last year or so. My pool has a bottom drain and a single skimmer on the side. My pump and all plumbing is original. The sand filter was replaced about 2 years ago. The pool is 16 1/2 X 32 1/2 and 5 1/2 deep. Liner is original with a few patches. I don't have a polaris or other type of automatic cleaner, just an old fashion vac hose.

But I have a problem with that vac hose. It worked normally until last year. Now, it will lose suction within about 30 sec after hooking everything up. It starts strong and then slowly loses suction. The hose is new (in fact I have tried 2 of them). The fitting in the skimmer is new. I always fill the hose from an inlet before attaching to the skimmer. I have tried turning off the bottom drain. The skimmer basket at the pump does not seem to be leaking air, although if I turn the system off, it will lose prime over time. I have talked to 2 dealers who don't have a clue except for saying it's the hose.

Anybody have an issue like this or a solution?

Thanks, John