FRT Delta Team Tactical Issue

FRT installed in Andersen lower. Function check is good. But rear retaining pin will not go through because upper receiver lacks about a millimeter closing completely. Tried the BCG from my other AR-15 with same issue. Have installed 3 position binary trigger on another AR-15 with no issues. I'm not an expert, but I have done other modifications and work on my AR15s with no issues. The Andersen AR is brand new and the FRT is too. What to do?