The pump may just not be powerful enough for the stone. Some stones are just denser than others. Pumps lose power over time, how old is your pump and what size is it? The diaphragm or other rubber part may be worn out, repair kits are available. Water pressure increases with depth requiring more air pressure, does your stone bubble if you hold it just below the surface? See if you can find a coarser looking stone. Sometimes a stone is just poorly made, the tube inside can be pinched or blocked.
From your description of events it sounds like your pump has lost power. First stone started clogging, pump started straining, new stone doesn't work. But check tubing for kinks and leaks. Put your finger over end of tube or crimp it shut, put as much of tube in water as you can with pump on. Check pump inlets for obstructions. I don't know if yours has a filter, usually a small cotton pad inside, check that, try running pump and stone without that filter, for a few minutes only!
That's about all I can think of. Minerals and algae or just plain gunk can clog a stone, sometimes you can clean them by boiling a few minutes and letting it soak a while, Soaking in water with plenty of vinegar in it or bleach in it can help. Or both but at separate times don't mix bleach and vinegar. Rinse well, especially if you used bleach.