I have two plum trees. One absolutely laden with fruit the other consistently around 4 or five plums. It is a blue black variety looks like Ile de Louvain but cant be sure and is in all other respects very healthy and grows/flowers vigorously. Could it be that it is not self fertile? Although I believe Louvain is.
The other may or may not be ‘Belle de Louvain’ which is self-fertile. Plums flower early and tend to suffer frost and weather damage, and many trees grow too leafy in our damp climate. Apply some potash to counter the vigourous growht and shorten whippy shoots in July and August.
This spring has been perfect for plums and perhaps a good set might be achieved this year.