Digital BW, The Print

I just bought an epson 2200, so I am a newborn to all those problems 
of printing bw that you perfectly know.

My first problem is very basic, I am unable to swap the light black 
and photo back carttridges. The light black one seems not to fit in 
the photo black slot. Would any kind soul tell me what I am doing 
wrong?

Thank you very much
J.
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