[ccyg8774]

Is your fake bolt in the right place? In some KA M4A1 they tend to be pulled out of its "rail" as you lift the charging handle to pass the little bump on the gearbox, then the bolt piece will prevent the upper receiver to fit in properly, just like the way in your pic. There is no easy solution for that, you will have to pull the charging handle as little as possible, and keep a finger in the fake bolt as much as you can to hold it in place.