I had this problem after opening last season, and battling an algae bloom. Couldn't figure out why pump wouldn't prime! Checked o-ring seals on both pump basket and chlorinator (which is now never used), etc. After many backwashes, my pool water level was a little low. Low enough to cause air to enter through skimmer. Once I added more water, actually put hose directly in skimmer, problem went away.

May your problem be that easy!

CaryB