Taller gearing ≠ more speed
Taller gearing ≈ more speed
To explain: You can't raise the gearing ratio to infinity. At some time you will have your gearing too high, and your ped won't be able to pull through, resulting in less top speed.
Could be that you've raised it too much, and now you go slower or the same speed.
I've had it myself a couple of times. I lowered the gearing and gained a couple of MPH on top speed and some more low-end because the bike couldn't pull through on the old gearing.