You should not have to justify why you wish a change of trainer/checker. A simple request in writing that you and that person are incompatible and that it will negatively affect your performance wasting training hours as a result. Any school or airline I know of will facilitate this, if they don't, get a union rep to follow up. If the company wants further information as to why, you can explain it then after they have made the change. If you are just a poor student it will reflect in lack of future performance with other trainers, otherwise the change should result in progress. If all the trainers/checkers in the place are the same LEAVE, and report the situation to CASA, it is not the place you want to be. As said before any punishment or ill treatment for requesting a change is workplace bullying, get the union involved and it will stop very fast.