Front feels vague mid-corner
Probably a lack of rebound damping up front so it's gripping, letting go, gripping again, etc. Add rebound damping a couple of clicks at a time. If this doesn't work, the problem could be a lack of control in the shock so return front rebound to where it was and add rebound at the rear a couple of clicks at a time.
Lack of rear end grip
Definitely a rear-based problem this and the most obvious cause is too much preload which tops the back end out. Reduce rear preload one turn, then back off compression damping a couple of clicks to let the shock work over its full stroke more easily.
Instability/tank slapping
Nine times out of ten, this is caused by a too-soft rear squatting under power and making the front light. First check rear preload and try adding a turn at a time. If that helps but doesn't cure it, add some compression. And if that's not doing it could be too much rebound holding it down too low, so try backing that off.
Bike understeers/won't hold a line
It's too high at the front. If your suspension's coping with bumps and front grip is okay,add rebound to hold the front down mid-corner. If that doesn't help try to get more weight on the front by dropping the yokes 5mm at a time, or increasing rear ride height with preload (providing you have sag to spare).
Forks bottom-out or chatter on the brakes
Add preload to support the forks more. You could add compression too. Check you're using max fork travel first and it isn't too much compression locking the forks before they've bottomed. If none of this works, reduce rebound - too much could be holding the forks down preventing them from working properly.
