I use PTFE tube as guide tube for thin pushrods in my sailplanes. The largest was 1/4rh scale. I have never (about 30 years) had the bonding (usually epoxy or Eclectic E6000) to the fuselage or wings loosen as you describe. 1) Harden the PTFE by stretching it to a limit. As you stretch it (including PE or most similar plastics), initially it is quite easy. Then it suddenly becomes much harder. That's often about 2 to 3 times the initial length. 2) Etch the surface for bonding. For some plastics, a soft propane/butane flame in the blue portion works. For PTFE, I use sodium dissolved in naphthalene and diluted with an inert hydrocarbon,* if needed. That's easily DIY, but similar etchants are commercially available.
*Sodium also dissolves in liquid ammonia (NH3), but the apparatus for doing that is more complicated. I do not recommend it as storage is difficult, and it is probably more reactive.. I have not tried it on Teflon.