A quick process to run through :
* Tighten the collet - if it's loose, no amount of burr swapping or cleaning will help
* Clean the collet - same as above, just next in line
* Be sure that tool shanks are clean, and the calibration pad is clean as well
* Verify that your tools if not factory set with a collar, that the collar is in the correct "window" of position on the burr. - there should be a "jig" that came with the burrs or maybe from Roland. We had a mill do the same, turns out the operator was willy-nilly tightening the collar, making it not seat properly in the collet.
* After above is checked, calibrate
* Check a crown; if that's no good and the burrs are new, you may need to replace the spindle. Not 100% sure on the life on those, but I know there is a limit. (50k or 500k hours maybe?)
I hope this helps out, and goodluck!
- Josh