HP laserjet 3700 Printing problem

I have a HP laserjet 3700 Printer connected to the network but last time i changed the Black toner, it would not print black. I mean it prints but its very faint and at the end runs out of black ink. The colors are OK . I can see all the color except for the black. At first it seemed like the toner was bad so i ordered a new one but the new one does the same. I have tried all the diagnostic on the printer . Please help! It has a toner collection unit that is full . Can that be a problem?