Lighting - Light Build Not Saving

Hi,
I’m using UE4 for my University project.
Here is my issue:
Everytime I build lighting, save, close UE4 and then reopen it, I still see the message on the top left of my screen ‘Lighting Needs To Be Rebuilt (x objects)’ etc, knowing full well that I have build the lighting only minutes before etc.
I’ve tried deleting my Build Data, which didn’t fix the problem. I am using sub-levels in my scene, but I’m unsure as to what end that is influencing this problem. There is enough storage space on my computer for these files to save correctly.
But after all that, I’m out of ideas.
Can anyone help me here?

once also I was experiencing similar and to have my project done I used advice from https://answers.unrealengine.com/questions/413041/lighting-rebuilding-however-still-getting-lighting.html