In the fundamental mode the stripline conductor is equipotential (every point in it is at the same potential).
If the top and bottom ground planes are not at the same potential, a parallel-plate mode can propagate between them.
If excited, this mode will not remain close to the region near the stripline, but will be able to propagate wherever the two ground planes exist.
The parallel-plate mode can be exited using slots, but its frequency dependence and excessive loss shall be taken into consideration.