My screen door can latch open, but not closed

There could be a lot of problems, but I can't really say without more information. The rollers being adjusted improperly could be one reason; that is to say, if one roller was adjusted high and the other roller adjusted low, then the unevenness would prevent the hook in the latch from connecting with the keeper properly.

But this is just one of many, many possible explanations. I would play with the adjustment of your rollers to see if that fixes it. If not, come back to us and give us a little more information on exactly what you're experiencing.