If you look in SYSTEM/DEVICES...?PHONES and find her phone, look at FORWARDING PATHS. Hopefully you see the voice mail path there. Open up her path by double-clicking the icon, you should see a bunch of parameters such as day, night, busy, no answer, including several call types. Make sure any behaviors you desire are on.
When I asked if the call BELONGS to that user, I'm asking if the call is her call, was it transferred directly to her extension. If it came via a hunt group or a forward from someone else SYS FWD doesn't happen, ever.