Hamstring injuries are common among athletes at all levels from amateur to professional. Once the hamstring muscle has been torn, it's more likely to tear again. Some athletes report five or six injuries on the same side.
Researchers at the Monash University Department of Physiology in Australia studied this problem. They found when injured hamstring muscles heal the optimum length of the muscle
needed for contraction changes. The muscle that was injured now operates at a shorter length. This can lead to microscopic damage from certain types of muscle action. These contractions are called eccentric contractions.
The answer to your problem may be to focus on eccentric activity. Instead of exercises to tighten up the hamstrings, it may be better to exercise while slowly lengthening the muscle. The eccentric contractions can shift the optimal angle toward a longer length in the hamstrings muscle.
An athletic trainer or physical therapist can help you with this type of rehab program.