How can I prevent the zipper pull from falling off?

The easiest way to put a broken zipper back on track is the cut a small slit at the bottom of the zipper on the side that is off track....

Make sure it's getting enough water and 6 - 8 hours of sunlight per day. You could also try giving it fertilizer once a week.

Check to see if you are providing optimal growth conditions. Does the temperature, sunlight, watering and drainage, and soil profile meet the needs of rhubarb outlined above?

This will happen if the paper you are using is too small so you should use a bigger piece of paper.

Improper installation and Plastic. Some "installers" just place the item on a flat surface without securing or tethering it at all. It takes just so many Garbage Trucks, Cement Trucks...

There is no need to change the zipper if the pull tab breaks. You can fix a new pull tab on the zipper.

Yes, you can install a regular zipper with a normal foot. However, if you want to master invisible zippers (and do other things like sewing on lace and sewing over...