When using coordinates, you need to specify a radius so that it knows how far to search from that specific point. Try changing your command to:
/testfor @a[x=-611,y=80,z=714,r=1]
You can also compact it down into:
/testfor @a[-611,80,714,1]
I believe that the reason is that without a radius the command block will not actually update its output, as the command block does not see the nonsensical command as either failing or succeeding.