Get a larger screw. How thick is the fiberglass?
Machine screws grab fiberglass better than wood or sheetmetal screws.
Drilling the correct size hole for machine threads is paramount.
One can cut a groove across the first 3 or 4 threads to help cut the threads, but a tap is better
Or get a self drilling screw with either sheet metal or machine threads, just be careful driving them electrically
I've tapped fiberglass 1/4x 20 successfully many times through as thin as 1/8th inch fiberglass.
Here is 1/2 inch of fiberglass holding an 800 Lb load.
https://www.youtube.com/watch?v=ZclHNSxr1xs&index=16&list=UUoPqTkOluQsuu3RpGnxVwFw
