@MB, have a look at these clamps as an alternative. The unit you used is heavy duty for sure, the link below has a distinct difference in that it has a spring effect that compensates for the compression of the rubber over time, so as the rubber get permanent set after being compressed with many cycles of heat and cold and the constant pressure, it will lose its elasticity and become just like a hard gasket. Maybe this is whats happened? These clamps have the spring tension around the outer and keep applying pressure regardless of the diameter changes. Might be worth a try? The only thing I dont like is the utililux style worm drive screw as these have been a weakness for my applications, but they have a recommended tightening torque so they should perform.
https://automotivesuperstore.com.au/...YaAi7sEALw_wcB