HP CLJ CP3505n only printing yellow

I have a CLJ CP3505n printer that is now for the most only printing yellow.  it will sometimes print cyan at the top and black at the bottom of the page.  I did notice that on the left hand side (facing printer) that there is a white plastic piece in the black cartridge slot that I must hold up whenver I have to put the black cartridge in.  Any ideals what is causing this and how to fix it.