A little blurb I read about Beinfang BufferMount tissue is that "Activates as low as 175 degrees Fahrenheit." So it sounds like you might be a bit cool.

Also it bonds while cooling -- not in the mount press. So I have a very clean flat surface I place the hot mounted print facedown on -- then rub the back of the matboard to provide pressure while it is cooling.

If the glue on either side of the tissue fail to fully melt, and if the print/tissue was not in tight contact with the matboard as it cooled, it might easily unstick when print and matboard absorbs moisture and swells at different rates.