The top screw on the sideskirt will not screw in. This is right where the mud guard is. I can push it back to normal but the screw will not fasten. I'm guessing the wheel well where it screws in is damaged or it's missing a nut on the back.

 

A picture is worth a thousand words.

A screw does not have a nut on the end.
A bolt has a nut on the end.
Not certain what is in your hand, but it is the key to how it is fixed to the body.
Nissan uses several versions of a spring clip. This type of clip works with a screw. No clip - no work.

 

 

Believe there is a spring clip behind here. It may have moved, fallen off or is damaged.

 

i had the same problem... look behind the skirt to see if the little bracket is not bent...

 

Did you just take off the screw and pull the side skirt?

edit Took off the side skirt and noticed the tab was bent, bent it back and also screwed into the spring clip and also popped the side skirt back into place. All fixed now. Thanks.

 

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Ima Take a guess and say there's no spring clip behind there but there is a plastic type anchor thing kinda like the white plastic anchor used for the front bumper. Sometimes over screwing will strip the plastic making the screw not get tighter

 

I don't see the edit button in Mobile version of this site but one solution is to use a fatter screw so it will rethread it

 

The tab that I was talking about has a little clip like the one in Davidv's pic on post #2...
Mine had bent when I was backing out of a driveway and the curb caught my front mudguard...