Clean it first. While you're at it if you want an eye open, screw the die back in your press after cleaning, leave the stem assembly out. Measure the O.D. of a piece of brass, size it, then remeasure the O.D. of the neck. It's amazing how much a FL die over under sizes a piece of brass. The button then has to pull back thru and open it back up. When you start getting a build up of resistance from the expander ball coming back thru the neck to open it back up, it usually because the die needs to be cleaned.Remove expander assembly, set die to size properly(lock ring secured) for the firearm. Replace expander and follow instructions for correct position. You will need lube, on the body, light lube neck inside and out. No lube on shoulder. Make sure the die is clean, rust free.
How many times or have you ever cleaned this die?
It could possibly be something else, but this is where I would start. At the end of this procedure I always end up knowing more about my die than before I started.