I am having the same problem and when I saw this reply I got excited thinking it would solve my problem, but my vertical one won't adjust up? Maybe it is already maxed out?

Hi symtha and Welcome to The Plumbing Page. At AskMeHelpDesk.com. You're responding to a 5 year old dead thread so I gave you one of your own. Look in then upper left hand corner of the first post form the date in the first post before you post, Thanks
First a few . Questions. Did the low level just start or has it been ongoing? You have a Fluidmaster ballcock,(see image). Correct?
When the tank's full how far down is the water level from the top of the white overflow tube? There's a small tube leading from the top of the ballcock to the white overflow tube. Is it connected? Answer me and I'll answer you. Tom