Nedbank poor property insurance service

I recently purchased a property with nedbank and they offered to do my property insurance so since I am their existing client for over 5yrs I decided to be loyal and do it with them. Now I realised that it was the biggest mistake. 2 months ago we had a hectic storm in our area and my retaining wall collapsed. I logged a claim and they took about 3 weeks to come out and assess then another 3 weeks later they sent another assessor. I waited a week after the 2nd assessor and called them only to find out my claim was rejected. I was shocked because it part of my property cover. They advised me the wall was not built to proper standards. This is certainly not my issue. I purchased the property with nedbank and they insured it so it was their duty to assess all the issues and bring it to my attention. I am highly frustrated and want this matter resolved.