1. Empty the cylinder of air
2. Remove the retention ring off the top of the regulator
3. Remove the plastic top piece, by using a wooden dowel
4. Remove the diaphram
5. Examine the lever height and adjust it accordingly (this can be unscrewed from the case and adjusted up or down with a small headed screwdriver that matches the screw head)
6. Once this assembly is removed, beware not to lose the burst disc that lies underneath it.
7. Once readjusted, reassemble, refill and check the operation
8. Repeat as necessary.
This should fix the problem you're experiencing, unless a part requires replacement.
Wayne