Odd, the screws that came with mine fit perfect. The head of your screw can’t fit through the hole? You do want it to catch on the plastic on the bottom, if it could slide through all the way the rail wouldnt stay attached.
In my case, two screws eventually loosened, which wasn’t a big deal, just bought better screws and double downed with wood glue.
The screws I bought are Everbilt Zinc Wood Screws Flat Head Phillips #6 x 1/2in. Great replacement, and if you dip the screw in wood glue or put a drop in the deck’s screw hole it will stay secure. Hope this helps!