The things that I would check are the temperature of the press and the pressure of the platen when it closes (clamps down) on the work. I don't know of a published criteria on pressure. The platen must clamp down on the work securely. The pressure on my press is such that I could not move a piece of 4 ply mat board when it is inserted and clamped down.

I use colormount tissue on my fiber prints (not recommended I know) but I have been doing it for over 20 years. This tissue is supposed to work best at 190-200 degrees. I have mounted on foamcore at one time and I did not have the problem you mention, so I doubt that foamcore is the problem.

I don't know which Seal press that you have. On my 210 M the platen does have an adjustment for clamping pressure (the two round protrusions that pass through the cross bar).

I hope that this helps. Good luck.