The first month is no excuse to be irresponsible. In fact, I'd think they'd want to make sure to pay the first month on time, to at least look like they're going to be responsible in future months! Starting off like this isn't good.

It sounds like they're taking advantage of you because you're their friend. That they're 27 and 28 means they've been adults for a long time, and should have been acting like adults for a long time.

I understand it would be really inconvenient to have to look for a new roommate(s), so I understand not wanting to put them out just yet. That can be a real hassle. But if it does come to that, I hope you have better luck finding a responsible and compatible roommate.

This might sound like a major step, but if they continue this behavior and end up not giving you money they owe you, don't be afraid to sue them. They're not being good friends in what they're doing, and you deserve your money back.

I hope it all works out for you.