I don't own this unit myself so cannot try it out. In looking in the User Manual, there's some info in the Troubleshooting section on page 184. Though I would have assumed that would affect the display of any zebras and not zebras at specific levels.
If you say crank up the gain/ISO, will zebras eventually show when at 90% or higher?
Maybe you'll need to do a reset of settings? Though I would strongly recommend first saving the current set of settings to an SD card for safekeeping. Though do note the settings that will ultimately not be stored. See page 117 of the User Manual.
Just caught a video that explained a bit about zebras on the C70. Are you shooting perhaps in Canon Log 2? If so, that seems to affect the zebras. Whereas if not capturing in log, then zebra settings above 85% seem to work.