My first thought is that you are putting too much oil on the valves.
My method is to withdraw each valve half way and apply ONE drop to each when taking the instrument out of its case wether is needs it or not. On a long gig I will oil at a break around every hour,
If one assumes a gap of 5/10,000 of an inch between valve and casing the volume to fill is around 1/100 milliliter,, one drop is approximately 1/30 ml so is ample.
Another thought, when in the case are the instruments sitting vertical or horizontal, ih horizontal oil might be migrating along the spring box to the felts.
Regards, Stuart.