Hi,
I recently changed my clutch and had this too, its just the clutch cable needs adjustment, it needs more tension. the clutch cable is on the left side of the bellhousing of the gearbox, it consists of two nuts. you undo the small nut which allows you to tighten the large one, you might need some pliers to hold the thread whilst you are screwing the nut. i have attached you a photo of the cable to show you what it looks like.
Thanks
Luca