Bought a package of Ideal In-Sure push-in wire connectors for use installing LED lights in garage. The package says they can be used on #12 - #20 gauge wire. I am using 12 gauge FMC on a 15-amp circuit - because I already have some. My problem is that the #12 gauge wire will not insert into the connector. I tried cutting on a bias, hoping the angled cut would help, but still could not push it in. Is there some trick I don't know or is the manufacturer claiming something the product can't do? Thanks.