If a class defines virtual methods outside that class, then g++ generates the vtable only in the object file that contains the outside-of-class definition of the virtual method that was declared first:
//test.h
struct str
{
virtual void f();
virtual void g();
};
//test1.cpp
#include "test.h"
void str::f(){}
//test2.cpp
#include "test.h"
void str::g(){}
The vtable will be in test1.o, but not in test2.o
This is an optimisation g++ implements to avoid having to compile in-class-defined virtual methods that would get pulled in by the vtable.
The link error you describe suggests that the definition of a virtual method (str::f in the example above) is missing in your project.