Zip up the G-code file and post it. The G-code should tell you everything.? If it says 15.6mm in the code and you have touched the tool off correctly then the length of the tool makes no difference provided it's longer than 15.6mm and you have Z travel enough it should go through the material.! If not then it can only really be slipping in the collet because the other tools are cutting correctly so it's not a mechanical or controller issue.