Is this a factory rifle?
This can be caused by anything from the feed lips not extending out far enough to retain the round in the magazine, to the incorrect magazine follower being used to to much spring tension under the follower.
Does the rifle do this if there is more then one round in the magazine or just when they are single loaded into the box.
On the feed ramp, what bullets are you using? Sometimes large HPs or soft nose bullets will hang up on the feed rail.
It is not a difficult thing to carefully low the feed ram to allow the bullet meplate to engage the ramp properly but you need to take as little off as possible to get the job done properly.
I am not sure if Wyatts makes a 223 class box and if they do I suspect it would be far to short for the 221 FB unless they offer a cernter feed design for the 223 class rounds.
I think some playing with your receiver as is could help you alot with the problem.
Again, is this a factory Rem 700 chambered in 221 FB or is it a custom rifle