Air into the reg > air out of the reg. So a leak within the reg would cause pressure and velocity to creep.
A leak between the reg and bolt would cause your tank to empty, but the reg would allow more air through to maintain the same pressure.
It is possible for a seat retainer/adjuster to back out, but they have o-rings on them which generally prevent it.
If this has only happened at a single practice, I'd blame the paint itself or changing conditions. Humidity affects paint size more than temperature and not every bag of paint is the same (especially practice grade paint)