This applies to the ISA 428

When syncing to Super Clock, the 'lock' LED does not actually light and should not be used as an indicator of lock.

If you can get a clean digital audio signal out of the 428 and into Pro Tools when set to sync to Super Clock, then you know it is locked. If you get digital noise, or nothing at all, check the sampling frequency and bit-depth settings are the same on both the 428 and in Pro Tools and set the clock source in Pro Tools to internal/Super Clock.