Hi,
Assuming you've checked the scope, rings, mounts, and rifle/action bolts etc and all is well then it could be one of several things. I shoot a 17 Mach IV and make all my own brass from .223 ( I need to get a life) so I know a little about getting these things to shoot.
Chamber length you can measure VERY accurately with a gauge that fits in the chamber neck ( get a mate with a lathe to machine one up). Its a "T" shaped button that butts upto the end of the chamber and slides into the case when the bolt is closed.
Trim a case neck down be 3mm too short and chamber the round.If it