The diagrams often used to show that virtual photons mediate the electromagnetic force usually give the impression that the exchange causes a push between the charged particles. But as we know, when the charges on the particles are opposite, they can be pulled together too. The 'Feynman diagram' looks the same in both cases, it's just that in the actual calculation so far as I recall it, there is a change in the sign of the factors that get summed together. For the details on why this happens, you will have to ask a physicist that does these kind of calculations, to give you a heuristic answer.