Leaking out of tampons after an hour... help!?

Well I wear the highest absorbency tampax tampons, and yes I do deffinately insert them right, but I notice I will start leaking out of the tampon after and hour or so, my flow is very heavy, but there are still white patches on the tampon where it hasnt absorbed any blood... And also I really don't need wider tampons, I have trouble getting them in in the first place. What could be causing this?