Could be other types of plants are uptaking your nutrients. In my warmwater pond that had an algae bloom of about 2 feet clarity last year it became clear and has stayed clear starting in late summer when after I took out about 100 lbs. of bass for harvest. At the same time filamentous algae took off last fall and now chara is dominant.

Have you seen an increase in macrophytes (leaved and rooted plants)?

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.