It's fairly common for it to take some time to settle back in after cleaning an air gun barrel.
This is is one of the reason why people don't agree on how often barrels should be cleaned. Some (well respected) people state that it should only be cleaned if you start getting accuracy problems. in other words, if it ain't broke, don't fix it.
Others claim you should clean every xx number of shots.
I use cleaning pellets every few months but I don't do a major clean unless I am having problems. Don't know if this is "right" but it works for me. You don't have to worry about gun powder residue with airguns. It's just a build up of lead in the rifling. You should be able to see that to some extent with a ST barrel.