The only thing that worked for me (and is still working) was to hydro flush the laterals. I used the Clog Hog hose attached to my 2,500 PSI pressure washer.
You should probably contract that out to a septic plumber so he can pump away all the gunk that comes flying out of those laterals into your D-Box. My laterals were literally all clogged up and there was no way that any amount of chemicals added to the drainfield could have helped.
After they have been hydroblasted, it might be worthwhile adding chemicals to the lechfield for the biomat.