When is the flight(s)? Is economy also showing as no availability?

You'll often see what you're describing when Y is oversold so they have blocked seats in F/C to cover. If nobody upgrades, then they will op-up someone from Y to F/C.

If you're actually looking in expert mode and seeing the upgrade fare classes also zeroed out, then it's just the website not showing the upgrade properly, and you'll end up being waitlisted.