Personally I've found that if I gently rotate the magazine tube assembly at the same time I'm pushing the forearm down/back toward the stock it helps pull the forearm into position much more easily.
Also, occasionally I've needed to depress the hammer back into the mechanism to get the forearm to fit all the way to the barrel before I try to lock it in. The small white dot on the side of the barrel needs to be right down in the alignment notch on the forearm.