I'm not the world's greatest bike mechanic but I like to figure things out. I have a triple that's been shifting fine. A few rides ago it was a bit hesitant shifting onto the small chainring. Today it refused to shift to the small ring at all. Back home in the repair stand I shifted to the small ring position and noticed that the cable went slack but the derailleur didn't move any further inboard. So it seems this is not a cable problem?

Trigger shifters on a recumbent. SRAM system.
Ideas?