As mentioned above, if you did install the volume restrictors, take them out again; they are actually intended specifically to reduce the velocity of your marker, and should only be installed when you are looking to shoot at very low velocity - when using Reballs for instance.
While the ST kit works perfectly well with normal paintballs - and is certainly still advantageous when doing so - one of the purposes of the ST kit was to make the Geo more compatible with Reballs. Amongst other things, lowering the HPR pressure and/or the dwell in order to reduce velocity lead to cycling issues. The volume restrictors are included to reduce the amount of air used to propel the ball, allowing for a lower velocity without having to drop the dwell or pressure.
If you want full velocity, you don't install the volume restrictors.
As Austin mentioned, dropping the ST kit straight in to your marker without increasing the dwell could also lead to a reduced velocity.