If you could post a picture, that MIGHT help; I don't recall my CMC trigger coming with anti-walk pins, and given that the 15-22 has a wider frame than a MIL-SPEC AR15, it is possible... POSSIBLE mind you, that you either didn't put the trigger and hammer pins back in, or they fell out because you didn't realize that they need to be there.

If the housing is indeed firmly attached to the lower receiver, then it would seem an issue with the trigger; breaking it in manually as suggested above MAY work.

I suggest you take it to a semi-competent gunsmith; if he tries to sell you anti-walk pins, ask him to install them (to ensure the correct width for the 15-22).

Good luck!