I'd grab a new cable and try again, but it might be more serious. I know that many of those left/front ST-5600 units had problems (which is why I asked). It was a near 'recall' situation where Shimano was replacing many of them free-of-charge. As I understand it, that's the last time Shimano made the 2x/3x and it was quickly replaced but a 5601 double-specific and 5603 triple-specific.
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"The older you do get, the more rules they're gonna try to get you to follow. You just gotta keep livin', man, L-I-V-I-N." - Wooderson
'14 carbon Synapse - '12 CAAD 10 5 - '99 Gary Fisher Big Sur