Digital BW, The Print

I have put a couple of cartridges of ink through my 1290 printer now, all onto Epson Archival Matte paper, and most of them complete rubbish. So what am I doing wrong?
I start with a colour image, scanned from a print, convert in Photoshop 7 using channel mixer on an adjustment layer, and then flatten to convert mode to greyscale. When I apply the gs curve to this file I get my best results but they are not something to jump up and down about. A lot of experimental prints hit the floor before there is anything of note to pin to the wall.
Today I started again, converted the file to greyscale and then back into RGB and applied the three RGB curves in selected rectangles on their own adjustment layers. The prints are awful. Only the part of the print that is not under a curve looks anything like a reasonable image. The rest are highly posterized and a complete mess.
Please advise.
John