Needs investigating. Something is jammed. Haynes manual will help.
the cable might be kinked. You need to be a contortionist and look under the dashboard where the heater unit is. See what happens when you move the lever. Undo the outer sleeve clamp and unclip the inner cable, then see if the lever on the dash moves further. If so, the problem is in the heater unit. If not, it is in the dashboard lever.
i suspect that undoing the clamp and adhusting the outer sleeve might help. See previous comments on adjustment.....