Well before I blamed the equipment I would remember that some times when guns sit a while they require a really good cleaning to get the residue fron oil grease old plastic or lead out.
Try the simple thing first,use boiling water one of the good citrus soaps and a new bore brush and scrub it good.
then look at your componants is the powder or sabot old? maybe they are not so good any more.
If its a wood stock did it warp?
If it has a scope on it maybe it got banged when the barrel of a scope has been bent so vary slightly that you can not tell it it will have a massive amount of parrallax.
Lee