Had a similar thing happen with my fridge a couple years back and it turned out the defrost drain line itself was clogged deeper inside, not just the opening - the drain hole in the back can look clear but the actual tube running underneath gets gunked up with food particles and mineral buildup. You might need to flush it from the inside by pouring warm water down there, or if that doesn't work, the drain tube might need to be pulled out and cleaned properly, which usually means calling someone or doing some disassembly yourself if you're handy with it.