While size 20 regular length snaps work for most projects, they aren't one-size-fits-all.
If your snaps are falling off your fabric, most likely you are working with thicker fabric and need longer prongs. Size 20 extra long snaps are recommended for thick material such as double layers of marine vinyl for snap tabs.
If your snaps aren't closing properly, most likely you're working with thinner fabric and need shorter prongs. You can simply cut the tips off the prongs of the snaps you already have to shorten them prior to installation. This will make them equivalent to our size 20 extra short snaps. If, however, your thin material is also stretchy or delicate, like knits, consider using size 16 instead for a more gentle grip.