Likely the upper edge of the clip retaining slot is too low, requiring that the clip be pressed very hard up into the mag well to get it to engage. If this is the case, you'll notice that the mag release requires a lot of pressure to release a full mag, and the problem is usually worse with a full mag: the last round presses hard against the base of the bolt carrier.
Solution to this problem is relatively simple: using a small file, raise file the upper edge of the retaining slot on the left hand side of the clip, about .050" at a time, until it slides into place. You can use a dremel tool or die grinder with a small heatless stone and then dress it with a file. I just clamp mine in my milling machine vise and use a very small end mill to adjust the slot.
good luck,
Shooter