It is maybe a beginner's trouble...
 

This is detail of my oil painting which I'm painting now. Yesterday, I draw rough underpainting, oil with turpentine.

Today, I add touches on it, oil color with mixed painting oil. Then this trouble comes. Like watercolor on oiled canvas, paint is not familiar with under painting. (See image. Today, I painted dark part.)

It is maybe a beginner's trouble. But I can't yet solve this problem. I want to know a cause of this phenomena.

Thank you,
Yoshiharu

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! :)