There can be a number of reasons why the Argon8M won't play 8 voices.
A common reason is if you use a stack 2 or stack 4 sound. This reduces the number of voices available.
Another reason can be a midi loop. This can be from a DAW or from the keyboard you use to control the 8M.
If you do experience 4 voices instead of 8 in a poly sound, then try turning the DIN in notes off in the settings tab.
Sometimes if connected to both MIDI and USB (for power) a doubling of notes can occur.
Making the icon grey means the setting is off.