What you did may fix it. Did you remove the nipple & flush the bore with water before oiling it for storage purposes?

Also... you need to make sure the place you are keeping your powder is a dry environment. Also need to need to consider powder age & how many times it's been outdoors on a range table. Damp weather will contaminate it within a few hours sometimes.

Also need to make sure your powder is reaching below the nipple. After pouring my powder "slowly", I twist & turn the muzzle-up rifle 180 degrees kinda fast a few times. Some folks use their palm & slap the side the barrel near the breech. Also need to consider finer powder FFF instead of FF.