I got this flash, and a new wide angle head for it.. still it seems to be one stop short of the numbers it gives at the back of the flash. I was wondering is this normal, or could i get the full power by relapsing the old capasitators with new ones?

I tested it at iso200, at 2.8m and it says f22 but metered f16, with the new old stock wide angle head and the zoom head that seems to be used.