Flat landings are usually the result of an approach that is either too flat, too fast or a combination of the two. Sometimes, using less than land flap may be too blame. You should really get this sorted with an instructor as a matter of urgency. Flat landings lead to landing accidents!
You may take your test with any examiner you wish; usually clubs have their own examiner(s) that they like to use but, you are not compelled to use them. There are no specific qualifications for a CFI, its just a title, some are examiners, many are not.