Sounds like short stroking to me. What happens is the bolt does not cycle far enough or firmly enough for the ejector to whack the case head and the case is driven back into the chamber. Weak ammo, insufficient gas, or heavy grease for lubrication can cause this. Mini usually apply way more gas than necessary so I'm wondering if you have an obstruction in your gas block. Or maybe your op-rod is dragging in the stock. Here's a test to see if something is dragging. Remove the op rod spring and install the rifle into the stock without the trigger group. Tilt the muzzle up and down at maybe 45-degree angles. The op rod should slide freely without dragging.
Could also be a damaged or broken ejector/bolt hold-open "finger". Does it show any signs of damage? It should be long enough to fill the slot in the bolt. If it's short the tip may have broken off. Not that I know this to be a common defect, just thinking of things that could cause erratic ejection.
When your rifle DOES eject do the empty cases kind of dribble out nearby, or are they thrown with gusto 20 feet away?