I'm puzzled by your results with ProFlex. In my limited experience, I have found it to be a superior, high strength glue/caulking, not just great at sealing joints, but adding strength to them.
The only idea that comes to mind is that I have found the shelf life of ProFlex caulking to be very short. It tried buying some ahead of time and storing it for a few months before getting around to my project. Bad idea. The product deteriorates on the shelf. I learned to plan my project, get it ready, then purchase the product, making sure it is not old on the store shelf.
I think I would try putting another bead of ProFlex on top of what you have. As you said, it has adhered well. Maybe a second layer will solve the problem.
Jim & Kate
2016 Creekside 23RKS, 2014 Ram 1500 Eco Diesel
1995 Monaco Dynasty 36', 2005 Honda Pilot