If the slave cylinder wont build up pressure and you've tried bleeding it and the master cylinder and the slave cylinder is good then what would make it not work?
a leak in the pressure line could do it. are you sure the seals in the cylnders are good? Is the slave not holding pressure, or is the master not building pressure? How did you bleed it? it makes a difference. Also, if the master or slave cylinders weren't bench bled before installation, it can take a very long time to bleed all of the air out of the cylinders.