I have a commode that will flush water very easily with a push or 2 from a plunger. However the next time it flushes, it doesn't have the force and doesn't flush completely. I have also put various drainer cleaners into the drain but nothing seems to provide a consistent flush. If it was clogged, wouldn't it stay clogged. I would think that once it flows and nothing solid is added, it should flush through. Any suggestions?