Geoff used to recommend this stuff:
Linky
I always carried a Clamshell repair - only works in certain circumstances, but likely to deal with a small puncture picked up in the course of a day afloat, such as coming alongside dock or hitting a sharp object.
I ALSO carried a small volume of thinners to clean up fabric before a patch, my experience was the same as your's, after some time in use, the patina that builds up on tubes prevents adhesion.