Sometimes you have to kind of push the follower into it's hole in the receiver and push down and forward simultaneously to get it to slide forward into place....
Make sure your stock is fully seated too...if it slips out a little when fussing with it it can cause a hangup getting it in there.
Also, make sure your magazine is fully seated...sometimes the lip can miss it's lug and go in awkwardly, yet seem correct...
I don't know if it's even possible to put a trigger in with the bolt open, but also make sure it is closed.