if you could publish a test site and provide a url we can see what is going on. I suspect though that this is the common case of there being too many menu items to fit - particularly if you have more items on the top level menu which is failing to display. What is actually happening is that they are wrapping to a second invisible line, outside of the top bar background.
The solution to this is to either:
- Increase the breakpoint (with a custom value) at which the mobile menu hamburger is shown
- Reduce the size of the title area to make more room for the menu items
- Reduce the number of top level menu items
- Reduce the text size of the TopBar menu items
This was covered in a recent post here and I made a little video which shows what is happening when the menu disappears. There is a link to it in my later post on that thread.