afaik that usually means that those line were optimized away or there is no debug data by some other reasons. Also, if you try to f11(step into) such code - you won’t see its internals.
I think the best you can do is to keep that in mind and make sure you debugging in “DebugGame” configuration, not just in “Development”.
upd: @Melvin_Brink, recently i’ve found another major reason that can break debugging: Increase the Number of Modules Loaded When Attaching the Debugger