Very common problem. It's because of the plastic which expands when it's hot. In the cold you have no problems. It's the reason I switched to the Pinnacle cap. I once had to call in the home assistance of the Dutch AA to have it opened. There a thread on this topic, I'll search for it. The new metal saddle is better, but the whole construction remains a PITA!
https://www.talkmorgan.com/ubbthreads.php/topics/816104/re-plus-fours-suspension#Post816104Here you can see the Pinnacle Cap fitted to my PlusFour, which is a relief!