A few possible reasons:
- It has already recorded that episode in the past 28 days
- The Series Link is marked as "New only" but it's a repeat episode
- The Series Link is set for a single channel but you are looking at a different episode
- Probably several other reasons I haven't thought of
Look in Recording Hiccups - it might tell you why it didn't record something. Or post some details of the series link settings, and that should make it a bit clearer.