No Plums This Year

The title says it all... we aren't getting plums this year, besides perhaps 10 or 15.  This is going to severely cut down on my garden poundage.  I'd be banging my head against the wall if I didn't have a gallon of chopped plums in the freezer waiting to be made into jam already.

How did this happen?!  The small new plum tree flowered first, and has a few plums on it, about the same as last year (though last year, all fell off unripened besides 2).  The large plum tree flowered, but I can only find one plum on it now.

The internet says there are a few possible reasons:

1. Lack of pollinators.  I have noticed that there seem to be far fewer bees around, despite having left all the flowering plants in the garden that the bees love.  The small tree is fine, so maybe bees were only out early in the spring?
2. Weather swinging from high to low and back again.  This a very likely explanation... our weather the past 2 months has been REALLY bizarre.  40's to 80's to 50's to 80's to 50's to 90's.
3. Too large of a trim, or an inappropriate trim (cutting branches in half instead of at the base) in the previous year.  Well... yeah, this totally happened when the landscapers putting in the stone raised beds decided to trim half the tree off (and fully remove the 4th of the tree that made better plums) despite us specifically telling them not to (which is how we got the small plum tree as a "please don't hurt me" gift).
4. Some older trees only produce every other year.  I doubt this is what happened, since the previous owners of the house said it was a big producer every year, and the last two years we've had tons of plums.

So, I am just taking a deep breath, fertilizing like normal, trimming the tree properly this year, and hoping for extra abundance next year.