Canon Speedlite 90EX Instruction Manual page 31

The slave unit flash does not fire.
• Check that the slave unit is ready.
• Move the slave unit closer to this flash.
• Point the sensor of the slave unit toward this flash.
The power turns off by itself.
• The Speedlite's auto power off has activated. Press and hold the Speedlite's power button
for at least 1 second.
The picture is underexposed or overexposed.
• If there was a highly reflective object (glass window, etc.) in the picture, use FE lock.
• If the subject looks very dark or very bright, set flash exposure compensation.
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