Navmesh generation and landscape holes Problems

As pictured in my attachments, you can see when i use my landscape hole material, the navmesh ignores the hole and wont generate, players can walk through the hold but the navmesh wont register it, does anyone know a way to allow the ai to walk in and out of my cave holes?

(before hole)
d25768274277d23f0a8220596aac694803679fff.jpeg

(after hole)

https://photos-6.dropbox.com/t/2/AAA184GCm4NbuvlseCjD6ZhwVNykM9RzltX9ZZsYlqT_fA/12/586060957/png/32x32/1/_/1/2/1.png/ELTg9NsEGKuiAiACKAI/T8__SEoRpBUVovJOTMMZueFa3EcuolQ8lGdkQk2Ppzc?dl=0&size=2048x1536&size_mode=3

Same problem. Did you find a solution?