i mean there kage has been sending people to assasinte Gaara for years, and none of them had ANY succes at all. it even states that hes never been hit before, i mean if rock lee can do it and hes not even a chuunin surley there must be some ninja from the sand that could do it.
so it leave sme to take one of two conclusions either a) the sand village is weak as hell (woudl seem to be the case due to the seemingly instant rebuffal of there forces following konohas will to do so) or b) he was just training him? could it be Gaara has misplaced his hatred in his father.