Weird.
I cut a Tmobile mircrosim to nano by using scissors and fit fine inside my i5.
Didnt have to shave or sand the back at all.
Its probably not cut right and sticks out of the sim tray on one side that's why it wont fit and slide in.
Cut it a little more by hand and see if it fits perfect inside the sim tray or there's a side that sticks out.