Hmmmmm, I've had this problem too and I don't know why it sometimes happens. However, there are several ways I've resolved it.
I've taken Windsor-Newton's "Blending and Glazing Medium" and rubbed a small amount onto the surface of my painting with my fingers - just enough to cover (really rub it in and wipe the excess with a lint free cloth) but not enough to make it "slippery." After this, the paint should stick.
The second method -believe it or not- it to take a raw onion, cut it in half, and rub it onto the surface of your painting. This will make the next layer of paint adhere without problems.
I usually use the first method because the odor of onions can be a tad too powerful on a hot day, but it really works!
Hope this helps!
