Question
What would make plum trees not bloom as much as they should.  For example, I have 6 plum trees on my property.  Three of them flower and look excellent.  The other three appear to be about at 60% fullness, but are also allot bigger.  You can really notice the difference when you look at the tips of the branches. They were all planted on the same day from the same nursery.  Any suggestions i.e. is there something I could test or add to the soil.

Answer
I would fertilize the trees with a fertilizer that contains less nitrogen than the general fertilizer. To encourage flower bud production you can apply a fertilizer that contains a small percentage of nitrogen, a higher percentage of phosphorous, and a little potassium. I recently purchased a liquid fertilizer with an analysis of 5-30-5, ideal for flower production. Because the product is sold as a bloom producer, the manufacture also added a little chelated iron, manganese, and zinc, all good for your plants as well.

Check with your local nursery for a fertilizer for increasing the blooms that contains more phosphorus than nitrogen.