Why could it be that when I polyurethane several items with the same poly and the same brush (and all items have similar texture) some of them come out smooth and some come out bubbly?

I do decoupage and I have a lot of experience with it. If you don’t know, decoupage involves the gluing on of pictures onto items and then putting on layers of polyurethane. Usually when I decoupage, I do one object at a time (a frame, a box, etc).

Today I am doing several items at once (Easter pictures decoupaged onto painted flat wooden plaques).

I am using the same poly from the same container, and the same brush for each. Out of four plaques, one keeps getting bubbly after I apply the poly, and the other three are smooth and correct.

Can you help me figure this out?