Shank size must equal collet size.

Do drop an "O" ring into hole before seating the bit. If you tighten a bottomed out bit the tightening action of the collet will try to drive the bit even further in leading to lockups or frozen bits when you try to remove them.

If you have to force fit or not able to easily tighten the collet check for a mismatch.

As for cutter dimensions, as has been stated, sharpen once and it has changed.

Straight cutter bits rarely match timber sizes so it is easier to tune the channel to what you have using a jig.