error: Class has not been declared despite header inclusion, and the code compiling fine elsewhere

You seem to be saying that the code you are showing doesn’t actually produce the compiler error that you are having a problem with. So we can only guess. Here are some possibilities:

• You could have forgot to include problemclass.h from the file where you are using ProblemClass.
• You could have misspelled the name of ProblemClass either in its own header file or in the place where you are using it. This can be hard to spot if it is a capitalization error such as writing Problemclass or problemClass instead of ProblemClass.
• You could have copy-pasted your inclusion guard #defines from one header file to another and then forgot to change the defined names. Then only the first of those two included header files would take effect.
• You could have placed ProblemClass in a namespace A, in which case you must refer to ProblemClass as A::ProblemClass if you are referring to it from outside the namespace A.
• You may be using templates and not expecting two-phase lookup to work the way it does.
• You could have misspelled the file name in your include. The compiler would not report an error on that if you also have an old version of that file under the misspelled name.
• You could have made ProblemClass a macro that only gets defined after you include problemclass.h, in which case what you see as ProblemClass gets replaced by something else by the macro preprocessor.
• You could have defined ProblemClass in a header file other than problemclass.h and then problemclass.h actually defines something else.