My only suggestion is you "dry" your mount board first before putting the print on it.

I always put the mount board into the press for a couple of minutes to drive out any moisture that may be present in the board. I then let the board cool down, and then mount my photos.

I was taught this 30 years ago when I first started dry mounting - our workroom was not temperature controlled, and the mount boards tended to absorb some moisture from the air over time..

I think we started doing it because we had a problem which from memory were similar to yours?