O O O O I know,
it could be one of two things if your pump is ok which I believe it to be
since there is no problem with the black and the blue, it will be the tubes from the capping station to the pump may be plugged, if it isn't that tube check the tube to the drain tank, that just happened to me the other day and it ended up being the latter of the two, I use a syringe and pull the ink through the head and the capping station and I removed the drain tube and pulled denatured alcohol through it till it was clean, and then ran a head cleaning session on med did a test print and back to normal...
//chopper