If you can't pull the bolt back at all, then no, it isn't normal.
If you can pull it back a little but not far enough to cock the hammer, then yes, it's a known phenomenon with the M4.
By the way, congrats on the new gun!
I'll play.
Not a Sig fanboy however somehow I have ended up with a bunch of them, maybe my Navy use of them after the Beretta 92 and 1911's. Only have one 320 I run in a Safariland drop leg rig; ~ 6K rounds thru this one. Started life as a VTAC flat 7 lb trigger and too big grip modules, purchased all of them, ended up with a Wilson Combat. Use Safariland holsters for all of them.