Possible your hardware or as I mentioned above, the tower you are connecting through doesn't support the increase speeds.
What would be helpful to check whether 3.6 actually works (have heard many a good response on the 3.6 service, but don't doubt your issues), would be to get at least 2 speed test results in at least two locations with 3.6 activated, and then deactivate it (wait a month as the service i still active for the remainder of the month in which you deactivate it) and do it again. This way there are multiple tests on multiple towers with and without the service. If the results are the same then well, guess its your hardware. Else , ould be the tower.
For a quicker option, call VC or ask V3G for assistance with the 3.6 service issues. He usually gets things sorted pretty quick.