SAU threats not to pay in case of claim for documents sent and resent to them

SAU keeps coming back to me on a 2018 cover I took and paid up till date that there are missing documents that pose a threat in the case of claims. I've sent and resent them and requested a confirmation but instead get a different individual requesting documents. I smell a rat. Why and how do these insurers manage to employ underhanded staff to do ***** executions in their favor, do they train them?

Very sceptical and frustrated