A number of things could have happened here. There's no decision making backlog so the most likely explanation is that there were issues with the assessment report and it's being reviewed. DWP won't be able to tell you anything because that's down to the assessment provider and they won't be able to tell you anything meaningful until any quality control review is finished. An MP won't be able to assist with the provider side either. There are a number of other possibilities but nothing to be gained from further speculation.
Best options at present would be to contact the assessment provider and ask them to confirm when the report was sent to DWP. If they can't say then you have your answer. It hasn't been... yet. If they give a date then you can go back to DWP and ask them what happened next.
If you're unhappy with their answer then you can lodge a complaint.