Why is it that every time I pick my nose it's full again in a few minutes?

The nearly-invisible thin mucus inside the sinuses is dripping down and drying on the nasal walls, so you have a constant supply of hard material deposited as it dries. If you rinse your nose with a mild saline solution (sold for this purpose), it may reduce the build-up. Antihistamines that counteract nasal drip may also be effective.

A humidifier might also help, because it may slow the drying inside the nose.