Thanks Passage for that description.
I went to a Hilary Vernon bitting clinic with my two horses mainly to get my youngster bitted correctly. My older horse I didn't have any problems with really but took him anyway. Hilary said the bit we were using - KK Sprenger - was slightly too big so she recommended one of her own range which was a hanging cheek with a lozenge in the middle.
Over the next few months his riding generally deteriorated. We had his saddle checked, visits from the dentist, physio, vet but nothing was found to be a problem. It was only after a visit from a very good myofascial equine bodyworker that we discovered the cause of the problem. She had us ride the horse bareback after his treatment - no problem, then with just a headcollar - no problem, with just a saddle and headcollar - no problem, then as soon as we put his bridle on he went all stiff and pottery again. Problem solved! I feel so incompetent as to not having picked up on this before.
Like Passage said he had become heavy and and reluctant to work and this was all down to the hanging cheek bit. Now he's back in his KK Sprenger he is okay again. So find out the reason why your RI recommended this bit before using it.