Home Insurance claim rejected

I have been paying for the Nedbank homeloan insurance since 2014. My retaining wall and the fence was affected by the recent floods in KZN. When I claimed from Nedbank, I was told that both the wall and the retaining wall failed due to poor construction. Why did I pay every month for something that is not of their expected level? Why was this not pointed out when they started insuring the property when the property was bought and the request not approved then? What have I been paying for all this time? I need answers from Nedbank. A very disappointed ***