Fifty dollars short three months in a row isn't a hint situation, that's a real problem. I'd be shocked if they didn't already know they were behind (like, the math isn't hard). Dropping hints just lets the resentment build on your end. You have to be direct and set a clear rule for the next payment, or it never stops.
Wait what? Three months in a row being short the exact same amount? That’s not an accident, that’s a plan. They’re testing to see what they can get away with. You’re totally right, being nice about it just teaches them it’s okay.