Jeanice

Sounds like maybe your Thinfire paper is curling.  Try cutting the paper to just 1/4-1/2 inch bigger than the glass.  Papyros doesn’t seem to curl.
Or, maybe you aren’t using 2 layers of 3mm glass?  Only one layer will pull in, dragging the paper with it.  2 layers will stay right there but round out the edges.  3+ layers will spread out.