It sounds as if the problem is, as Tim says, a pressure problem more than a leak problem.
My boat will go from sagging tubes to reasonably firm with the change from night (50-ish F) to direct sun (70's F). If I leave it at operating pressure (2.5psi is the spec), the sun will greatly overpressurize the tubes, blowing seams.
If your repairs are not holding, this may well be your problem. Check your pressure when the boat is cool in the shade, then again when it's been in the sun for a while. Eventually, you'll get a feel for what state it should be left in to ensure it remains within the pressure specs.