Russ as a general matter ponds are short (if at all) on P not K and sometimes on N. Iding the missing element N=nitrogen, P=phosphorus, K=potassium , can be critical in the missing link. Other things are also key - like water temp and alkalinity. Your alka is good. It sometimes takes a while before the soup is ready. We have one pond that still does not have a visible bloom even down here. It is only missing time at the right temp - the rest is I know ok. If your watershed is similar to your pond area (as most are) then the same data is in your soil samples test wrt NPK and others. Further bottom aeration can slow down a plankton bloom.