I use the FS most of the time and switch to the closest player and usually get user tackles. I also shift my D-Line and LBs according to the strong side of the offense. I've been using defensive hot routes to get my OLBs more involved in the pass rush too(straight down to blitz/pass rush).
Another tip that may help your LBs get more involved is to use the FS to help support on run plays(moving him in the box, usually over the TE/strong side), and use strafe to shadow the HB, and usually that will help free up my LBs, and also seems to allow the LBs to shed blocks faster. Almost like the CPU is more focused on getting a block on me.
Oh yeah, I also use the Pass commit when they pass, and that seems to help get my players in better position to make tackles. I never use the run commit though(instead I will bring 8 in the box, sometimes 9 to stop the run).
Hope that helps!