OK , a LOT depends on how mechanical you are, but if the Velocity adjuster isnt making any difference , ,it might be a restriction of the gas passages before it gets to the adjuster , or even your tank, have you tried a different tank yet ? , if you have eliminated that as a possible cause .

try disassembling your marker carefully , and inspect the gas passages, (and your O-rings while you are there ) for blockages , bits of teflon tape , etc that would cause not enough gas to make it thru your marker ,if you cant see thru a port , take some Hot water, or electronic cleaner (be careful with this stuff , some spray cleaners will attack the rubber in orings , so will WD-40 ) and blow it thru to verify good clear path for gas , also look for areas of considerable wear on your moving metal parts ,if you can feel a difference by rotating the part ,its likely to be causing some degradation of performance, try reassembling the pieces to where the worn areas arent rubbing the same places they were (rotate them 180* )

Also , when you reassemble your gun , coat the orings with a very light coating of grease (Vaseline , or preferably automotive chassis grease ) to help ensure they seal up when they should .

Hope this helps .

Edit : use the grease sparingly, but if you do get a bit too much in your 68 , dont sweat it ,it will just blow out, worst thing that would happen is foul your barrel with lubricant . (time to clean your barrel !)

THe problem is probably that its too cold and ur co2 tank is cold.  Sl 68 is especially sensitive to this because it doesn't have a botttomline.  Try keeping ur tank inside and warm and then shoot again. COld tank means low fps while hot tank makes higher fps which can be corrected with a velocity screw.  If problem persists, contact tippmann or the guy u bought it from