Staples won’t go through

When stapling the uni-trim, the staples won’t go all the way in & end up getting crushed & crumpled as if the plastic is too tuff. I’m using an ele stapler gun.

Sometimes they go in but not all the way. so then I tap them in further with a hammer. On the box of staples it says heavy duty. Why is it so hard to put a staple through the plastic? Same thing with a hand stapler. Curtis in Florida