leading roundhouse

[akira]

hi all im struggling a bit with the execution of lead roundhouse kicks from a left (leading) stance, whilst i can do them at head height from a right(leading not rear) stance, knee up, pivot, kick ect from a left stance im unable to extend the bottom of my leg fully, it seems a muscle above my knee becomes extremly tight and i cant straighten it fully, its only seems to be the muscle above my knee/front of the bottom of my thigh, my groin and hip area are okay, is there an excersise or stretch i could do to strenghthen or stretch this muscle?