I have installed several Alpha rails. Before that, it was TRX and VTAC-TRX. The TRX series used a proprietary barrel nut (at least the wrench was included) that people complained about. One of the improvements with the Alpha was that you didn't have to remove the barrel nut to install it if you were steady with a Dremel.
The Alpha seems to be a better overall design, but is a bit harder to install to the std barrel nut. TRX was easy to install once you swapped barrel nuts but was also easy to over tighten the mounting hardware.
When mounted, does the top rail meet up with the upper receiver rail? That's the picture we need to see to try and help troubleshoot. Regardless, I'd take Stefan up on his offer.
Hope you get it worked out. I like my rails--Alpha and TRX, even a VTAC for good measure.
ETA: I read over your last post again. Are you putting the "leg" on the clip towards the muzzle? They shouldn't slip or push away when tightening. Your picture of the inside of rail shows one that looks correct.