I'm going to concur with the recommendation of using something else to start ripping the sod/grass.
Sod/grass is a bit like fabric since all the roots tend to intertwine and create a pretty resilient mesh, and since a rear blade (or even the blade on a box blade) is a bit like a safety/cartridge razor in how it works ....well, you're seeing the issue. So something to tear-through or otherwise cut the roots is needed. Scarficers, rippers, middlebusters, coulters -- really just about anything that primarily shears vertically along a path parallel to the tractor's path should work.
Of course on the flip side once you get the blade under the sod's root layer it'll likely stick together and roll like a fabric as well...
Actually that's how a nearby sod-farm harvests the sod -they have an implement that uses a pair of coulters/disks to cut the width of the strip, combined with a blade sliding under the roots which creates a roll of sod (that looks like a large roll/bale of carpet/paper). That role is then transported and sold as is, or further sliced into shorter lengths.
