Lets start with the easy things to check then move on to the harder ones. First take a screw driver or like object and put it in the latch hold the handle open like your trying to open it when it is closed. While holding the handle open try to open the latch with the screw driver. Some times it will close and it need to be in the open position in order to grab the striker.
Let me know if this helps and the results.
Try adjusting either the latch or the striker. They may have moved too far and no longer are close enough when the gate is closed for the latch to meet the striker. If you still need help just reply.