Hello Lorraine:  In my research, I found the following top 3 reasons why a plum tree does not produce fruit.

1: Over fertilization or over pruning.

2: Frost damage.

3: Poor Pollination.

I'm not sure if you fertilize your lawn or trees, but if you do - do you fertilize the lawn
surrounding the trees? Fruit trees do not know that you are applying nitrogen only for the grass. Rain can move the nitrogen down past the
grass roots where the trees can take it up.

You could have the soil tested to see if there are any nutrients the trees might be lacking in (or if there are nutrients that are too excessive that fruit trees do not thrive in) - http://pss.uvm.edu/ag_testing/?Page=soils.html

You do mention that you prune, but perhaps you are over pruning?

Frost damage is certainly a possibility since we live in Vermont.  You do not say what variety of trees you have, but there are cold hardy varieties that can be planted to avoid this. Also, the area where they are planted can play a role in frost damage. 

Poor pollination - yes, that is a possibility.

Here is a link to solutions for all of the above mentioned issues - https://extension.psu.edu/home-orchards-why-is-there-no-fruit-on-my-tree

Also, here is a link that talks about cold hardy varieties - https://extension.umaine.edu/fruit/growing-fruit-trees-in-maine/varieties/

While I cannot determine exactly why your trees do not bear fruit, I hope the above solutions might help you narrow down what's going on, and you are able to enjoy a bountiful harvest of fresh plums in the future.

Best, Lisa