The lever feel any weaker ?
Id say give it a cleaning. Thats always first with guns acting funny.
Can you change the gun to discharge the bottom barrel first? Try that after the cleaning then the top see if it pops open.
After that there are a multitude of things id be checking in the shop. Lever spring, the locking blocks, and a few other things.
Is the gun still tight on face? Specifically, with the action closed, can you get the action to open slightly by manipulation of the barrels and forestock?