Nav mesh help please

Hello all,
I am having this strange issue where i have to keep rebuilding my nav mesh once i leave that level. im currently using 4.21 and im wondering if anyone else is having this issue or have i missed something ? Once i build the mesh and press “P” its green and shows me its all good i then build and save the map once i leave that map to go back to the main menu map and then return back to that map the nav mesh is there but if i press “P” with the nav mesh selected it wont show up green so i delete the nav mesh make it again and i am back to the start please help