This will depend on the situation, however if you find that a person simply needs training and mentoring then this is likely to be within your capacity as an employer. If the employee is struggling to cope with a changed process, where previously they performed well then you really ought to make every effort to assist them through this process. If however, after all reasonable efforts have been made to assist a person perform to a satisfactory standard, and they are still not meeting their targets or agreed performance standards, then you are within your rights to consider the termination of the employee’s contract.