Collet could be worn and needs to be cleaned and or replaced.
Check if collet is put on correctly, and is tight.
Run z-zero routine, sent the Z axis to zero and verify that the bit reaches the top of the material. Then, mark the shank of the bit where it meets the bottom of the collet, now run a test cut below 0 and see if the bit moves up in collet during the cut. If it does, the bit has slipped, replace the collet and collet nut and ensure proper installation.
Check that spindle is tightened to its extrusion, and square.
Make sure the lower wheels of YZ car are adjusted properly (there is a document on how to do this under maintenance on the website)