Primary Menu ~Unchecks itself

I’m adding pages to my custom Primary menu and after I arrange them all in menu structure I make sure my Menu Settings are set for Primary Menu
when I click Save Menu after it refreshes it doesn’t save my changes and My Primary Theme Menu is no longer check marked off ? This makes my whole site menu change ? I’ve tried this in Google Chrome & Firefox but the end results the same- Pages in my Menu aren’t saved ? I have to go up to keep re-checking Primary theme in customize–navigation under Primary Menu ? Thank You in advance