Bore fouling has a definate effect on shot placement, as does the barrel temp. When I shot smallbore in college we only cleaned our bores at the end of the season, as anytime we did our zeros were thrown off and accuracy did not return to normal for about 50 rounds. Also we always fired 5 rapid shot before begining practice or a match to bring our rifles up to operating temp, otherwise they would place the shots about 1/4 inch off the point of zero. Either or both of these may cause the effect you are noticing.