There is a reason the Wby MkV is not more popular with accuracy minded gunsmiths, there is only so much you can do with them and at times you will get one that just will not perform as it should.
I personally do not distain the Wby MkV but it is not on my short list of preferred actions that I recommend to my customers for one of my custom rifles
That said, if the receiver is true and a quality barrel is fitted properly with a correctly dimensioned chamber and throat and everything is sitting in a stress free bedding, they generally shoot extremely well.
First thing I would do it clean the bore as mentioned. This solved 50% of accuracy problems in fires like this.
I mean clean down to bare metal for the full length of the bore, not just the muzzle, just ahead of the throat is a major area of concern with fouling so make sure the entire length of the barrel is clean.
Once it is clean and accuracy is no better then I would start looking into bedding issues and if that does not fix the problem I would recommend either cutting your losses and selling the factory rifle or rebarreling it after being accurized.