Assuming its stuck in the ramrod channel.
Pull the barrel I bet you money the "U" spring has flipped over & is snagging on the end cap.
If it isn't then its probably been allowed to fall out when pulling the lock for cleaning & installed upside down on the cross screw.
It should be reinstalled facing back towards the butt &with the curve down.
__________________
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”
