Buybox visibility has suddenly gone

My client switched to FBA on Amazon’s request to improve sale and visitbility. Up until then they had regular sales using Amazon ads.

However, they received some bad feedback and can’t seem to generate traffic anymore for the last 3 weeks using the same ad campaigns they have been running for the last 12 months.

The buybox visibilty has gone. What is the tasks you advise to get this turned around?