If nothing seems to stick to the roof it probably means the previous owner waxed it or used some type of silicon based product to wax/shine it up.
Your probably going to have to strip the stuff off to get any type of adhesion with sealing products.
Find an automotive paint supply store and get some wax stripper and start going over your roof and seams. You may have to do it several times to get a perfectly clean surface. This applies to the fiberglass area of the roof. Don't know what your roof is made of, but if it's fiberglass thats the way to go. If it's a rubber roof or coating, someone else may have some advise. Got to have a clean surface to start with. Then you can apply your caulking/dicor/eternabond tape.
Grumpy