Filter losing suction

[Atomic]

I have a sand dollar filter, going on the 5th year now. Replaced the head last year. This year I replaced the sand and started to get the pool ready for summer. The filter seemed fine... but everytime I vacuum after about 3-5 minutes I start to lose suction. It comes back after I flush the filter and then go back to vacuuming.

Anyone know what could be causing this? I am afraid I might be stuck buying a new filter this year.

[Poconos]

Welcome to the forum.
Some questions.
--What kind of pool, just curious.
--When you lose suction does the pressure at the filter rise? This would indicate the filter is plugging.
--When you lose suction are there air bubbles visible either in the pump basket or coming out the return(s)? Tiny crack or leak on the suction side somewhere.
--Is there a lot of crud on the bottom that you're vacuuming? Plugging filter.
--Finally, does the system run fine in the normal configuration i.e. using the skimmer and not vacuuming? If it only has a problem when vacuuming then I'd suspect a crack in the vacuum hose.
Best I can come up with for now at this later hour.
Al