Effort during the session can also lead to discomfort. Effort can be any attempt to "do it right", or to try to think the syllable clearly throughout the session, or to "try" to reach a particular state of mind, and so on. Effort can be subtle and one might not even be aware that one is trying. Even trying not to try is a form of effort.
Gavin, Long-term TM meditator
Cape Town