14avq won't tune up on 20 mtrs
 

I recently purchased a hygain14avq because I wanted a multiband vertical, primarily for 20 meters, but also for 40.
I have tried to get this thing to tune up on 20 (after getting 10 & 15 close, but have had little success. 10,15,40 all resonate high (antenna too short), but 20 resonates low out of band.
I tried to shorten the first radiator (a lot!) to help 20, but it didn't seem to do as much as I thought it would.
My ground is about 3.5 ft down (I know its not great but all I could do) in very wet sandy soil and does cause 20 to have a pronounced dip but at 13.4 center and about 350khz wide at the 2.0 swr points.
anyone have any idea on how to make this tune higher in freq?