Usually I forget to lime the plums early enough. Too early, it will just all wash out of our thin, open soil; too late and there won't be enough readily available to the roots by the time the fruitlets are forming. I'm sure this is the reason that plums are so disappointing here. The wild plums only produce prolifically when they reach 12' plus; local sloe bushes hardly produce any fruit at all.
Limed the area around both about a month ago, and spread calcified seaweed around the root area liberally today. Fingers crossed for a better year for plums in 2011.