Second Thunderbolt Display not working?

i have 2 thunderbolt displays , 2011 15" macbook pro w/ thunderbolt. when 2 displays plugged in, 1 recognized. tried switching around cables, installed software updates, nothing has worked. suggestions?

 

figured out. not enough power both of them. unplugged imac next them, , worked fine. plugged imac in , turned it, monitor stopped working again.

 

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