Impeller not pumping water

I'm experiencing issues with my Volvo Penta 5.0 GL engine's impeller not pumping water. We tried submerging the intake completely in a bucket of water and running the engine briefly with the impeller hose disconnected to check for water output, but it didn't pump any water. Even with a brand new impeller (and the same issue with the previous one), we couldn't get it to plinko work. We confirmed that the intake isn't clogged because we were able to pump water through it manually. I suspect the impeller might not properly fit the casing, possibly being too small, but I'm unsure. If anyone has any insights or suggestions, I'd greatly appreciate it. Thanks.