Is the leakage on the outside of the mold or internally. Possible causes for external leaks are worn probe seals or improper cleaning when being maintenance. Internal leakage may be caused by shooting through when manifold in not fully up to temp or wear on the crush rings reducing the preload on the system.
There are several Hot Runner systems on the market and many causes for leakage. It would be helpful if you could be more specific with the type of system and location of leak. Also a great source for trouble shooting would be to contact the hot runner manufacturer since they are most familiar with their system(s) and the problems they may have.
I have seen hot runner internally flooded with plastic because injection parameters and settings are wrong. For example: mold cavity fill time too long, which causes the material to freeze up close to the injection points; then hold pressure applied onto frozen injection points—> the molten plastic has to go somewhere, maybe into your hot runner. A good symptom for this problem is if you have material leakage at the main mould entry valve gate and plastic creeping back up your screw cylinder – because the molten material cannot get into the frozen cavity.
One specific situation:
if your hot runner has transfer gates to inserts, and these inserts are not currently in use. The transfer gates might need time to warm up to the same temperature, so they stay shut. Or check if these unused transfer gates are really well shut.