shield doesn't protect me when i crouch

So to use the shield I have to crouch, and lately i'd say 95% of the time when i crouch, the shield's position doesn't move to like in front of my character (so I still take damage from arrows and creeper explosions etc even when i am supposedly "using the shield"). 

I don't really know what triggers the glitch or what fixes it, on occasion it'll fix itself but it never lasts very long :/