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I believe that this "sinking in" dulling effect often seen with blacks and other colors is because the black paint is losing its oil to the layers underneath. If the lower layers were thinned too far they pull oil from the top layer. The top, dull layer is then left without enough oil to even bind it properly to the painting. You can see the result of this when you wipe a dry cloth over a dry area of dull black paint and some of the pigment comes off on the cloth: it's not bound properly to the layers below it.
The method I learned for solving this problem is to restore the oil to the upper dull layer by putting a very thin coat of linseed oil over it. This is called "oiling out". Let the linseed dry and soak in. A few days later you may see that parts of the dull layer still need a bit more linseed, so reapply. When you're all done, the top layer should have an even glossy sheen like the rest of your canvas and also be bonded properly with enough oil to hold it to the painting.
Can anyone else on the forum let me know if I'm off base here? I've had this problem a few times but by not thinning my paints so much, and by using this linseed "oiling out" method, it seems to have solved the problem.
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