How often does this happen and what solutions have you tried?

I like listening to problems that people have and trying to understand how people are thinking about solutions. 

@Anh Tran how often does 'what' happen?

Oh sorry, the question was in the title. Multipart question!

What’s the hardest part about investing in rental properties? And how often does the hardest thing happen? What solutions have you tried?

collecting rent from different tenant since they use different methods. 

The hardest part for me is dealing with tenants who have clearly lost touch with reality and stop communicating. I've talked about this before on Bigger Pockets, but this is the standard I use for starting an eviction.

Hardest part?

Probably the concrete.

After that, the tiles.

Cast iron is pretty hard too.