Why are some visual stimuli consciously detected, while others remain subliminal? We investigated the fate of weak visual stimuli in the visual and frontal cortex of awake monkeys trained to report stimulus presence. Reported stimuli were associated with strong sustained activity in frontal cortex and frontal activity was weaker and quickly decayed for unreported stimuli. Information about weak stimuli could be lost at successive stages en route from visual to frontal cortex and these propagation failures were confirmed by microstimulation of area V1. Fluctuations in response bias and sensitivity during perception of identical stimuli were traced back to pre-stimulus brain-state markers. A model in which stimuli become consciously reportable when they elicit a non-linear ignition process in higher cortical areas explained our results.