A worn slider or pull is a common problem that sure can be annoying by allowing the zipper to pull apart and not stay closed.
One thing you can try is to “pinch” the slider. Assuming you can still move the slider along the zipper you’ll first need to open the zipper all the way. If it’s jammed and won’t budge you might have to pry on the slider with a screwdriver to free it up some being careful to not break any teeth.
Once the slider is positioned all the way down, grab a pair of pliers and pinch or squeeze the slider on each side slightly, attempting to give it more grip on the zipper. Don’t pinch it too tight or it will bind. Test the zipper to see if it will stay shut and if it won’t you may need to pinch it a little tighter. This has worked for me many times on many different tents but if the slider is shot then you can try fixing it yourself with a Zipper Repair Kit.
Good Luck!
Chris