Is the paper making contact with the cloth/sponge? It looks like it's lifted up, all around the edges. Cut the parchment smaller, to fit in the tray, so it lays flat. It needs to be in constant contact, not just pushed down occasionally.
If that's not the issue, it looks like your paint is separating, which would be due to diluting with too much water. Basically, when you thin it out that way, eventually the binder fails and the pigments separate. How much water are you mixing in to your paint? If that's the issue, you just need to start thinning with an acrylic medium as well as water, so it stays homogeneous.
Also, washcloths are meant to absorb water, I guess sponges and paper towels are too, but I don't know how well a washcloth would work, probably not well. So little of the cloth will make contact with the parchment as it will just sit on top of a couple threads.
That's all I can think of at the moment, good luck and nice necro.