RE: Glueing stuff inside a fiberglass fueselage??
You could be having a problem with mold release on the inside of the fiberglass. I am currently finishing up a Grumman Panther that had lots of mold release on the inside. I thought I had cleaned it all out using lacquer thinner, acetone, etc. I found out that these solvents don't really affect the mold release. The only thing that works with this stuff is very warm soapy water.
If there is even a trace of mold release in there, nothing will stick to it for very long.
Once you are absolutely certain the surface is clean, then I second the recommendation for Hysol 9462.
Another adhesive with superior bonding qualities is E6000. It can be found in craft stores. Locally there is a Michael's craft store that always stocks it.