I've never used those particular fittings (though I plan to in the future). Perhaps they sent you the wrong size tubing or the fittings aren't the size you ordered. I had major issues trying to use Tygon 1/2 inch tubing on my Koolance fittings. Turns out Tygon 1/2 inch has very different dimensions than vinyl tubing. Best I can say is try ordering a foot of various different tubing sizes and insist they mark exactly what is what so you can find what works.
Performance-PCS is usually pretty good about making things right, but sometimes you have to be persistent with them. I know there is one guy in their customer service dept that is a real asshole and refuses to bend, but eventually you get to their manager who always wants to make things right.