Have you tried messing with the settings in your Project Settings->Navigation Mesh? Often you’ll need to increase settings such as Agent Max Height, Agent Max Slope, etc in order to get the Nav Mesh to generate over a steep surface such as a staircase or ramp.
Let me know if adjusting those settings makes a difference.
I am marking this topic as resolved for tracking purposes, as we have not heard from you in a few days. If this issue persists, feel free to respond to this thread. For any new issues, please create a new Answerhub topic.