Other than OzzyJ's suggestion .....
If a barrel really, really isn't fussed by cleanliness then it shouldn't make a difference. fat chance but I suppose there are such.
Having a reliable cleaning ritual (that doesn't require dew collected on the Equinox after the dawn sun's rays have cleansed it..) is a good reference point. Then, if they don't group within the first 20 shots I would try a different batch. If they did group after 100 then so what, keep looking for the ones that are sweet from the start and maybe they will open up after 150 or 500 but you know you can clean and reset.
A quick half-hearted clean puts you in unknown territory.
Be a bit careful what fluids you clean with if there is any chance of it dropping down inside i.e. down the hole where the air comes up. A small amount on a pull through felt does a better job if repeated than a bucketload done once.
Simona is right - a proper clean and the right batch takes very few to hit the aim point.
BFTA Secretary 2012-2014
CSFTA Secretary 2014-2016