Object listed in headerfile, yet undefined reference in cpp file

In the class, in my header file, in the private section of the class I have

 Heap_PriorityQueue<Passenger> oldBoardingQueue;

I have #include "Heap_PriorityQueue.h" in both the header and the cpp just to be safe. I haven't even started using it in my cpp file, yet when I try to compile the cpp file throws up a bunch of

undefiend reference to 'Heap_PriorityQueue<Passenger>::(insert function for Heap_PriorityQueue class isEmpty, add, etc.)

Followed by several

undefined reference to 'non-virtual thunk to Heap_PriorityQueue<Passenger>::(Heap_PriorityQueue functions again)

Unsure of how to proceed. Am I declaring incorrectly?