How can i fix my screen on the sliding glass door so that it closes all the way if glass door is open?

The screen at the very top is still open when it appears to be closed all the way allowing bugs & flying insects to get in the house, I can’t see what they problem is because only the other side of the screen (that doesn’t move) is on a track. I don’t know if I should just buy a whole new screen or if it’s fixable. I’d like to be able to leave the glass door open when it’s cool outside vs. running the A.C. but as of now that’s what I have to do because I don’t deal well with creatures of any sort..