No effect is applied to the headphone sound.

The following FX aren't applied to the headphone sound. This is part of the unit's specifications and not a malfunction.

■ SEND
BULILT-IN, EXT1, and EXT2
■ BEAT FX
DELAY, ECHO, PING PONG, SPIRAL, REVERB, and SHIMMER

For example, if you apply an effect to the channel 1 sound, the effect won't be applied even if you monitor the channel 1 sound with your headphones.
You can check the sound of BEAT FX by monitoring the MASTER.
You can also check the sound of SEND effects by monitoring the channel receiving the post-effect sound (the channel where [BUILT-IN], [EXT1], or [EXT2] is selected with the input selector switch).