Digital BW, The Print

Hello,
I've happily printed both BO and QTR on my 2100 ... but I've just been trying to do a BO print and what appears to be happening is that the printer is only laying down the black ink that it would if all the colours were being used  - so not much ink is being laid down at all - only the darkest 25% or so.

I am using exactly the same BO settings that I used before. The Adobe Photoshop driver is set to Gray Gamma 2.2 as it would be for BW prints on my HP 7660.

Can anyone please give me an idea of what might be wrong? 

Many thanks in advance,
Richard
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