Not sure if it is true, but the video has pointed out that Castle is dealing with a left thumb injury since late December. I also noticed that there was a bandage thingy on Castle left hand for a few games already.
The injury might be the cause affecting his shooting. Dunno why the Medical Team is not resting Castle...as a thumb/hand injury would affect shooting, passing, layups etc... But then again, Fox also play through his finger injury last year and still managed to drop 60.
Maybe it is up to the player's pain threshold and if it affect his performance.