this has happened to me multiple times in the past, the line rots/cracks within the pump. machine can continue printing normally due to the siphon-effect of the ink, but the pump isn't actually doing anything but making noise and the waste-line is merely a cracked slide that allows for most of the ink to just drip-down into the waste container. The rest may be all-over your ink pump, and the "trap" beneath the pump.
i'm not 100% certain on your fix, without replacing the pump, there's some "fiddling" you can do to try to resolve the issue.
I have also resolved this in the past by simply depressing the capping station head repeatedly while flushing the lines with solution.
(i don't know exactly the reason why this fixed it but it did long enough for me to run a nozzle wash. hah.)
another option, for a temporary fix, would be to get a surgical clamp and just pinch/clamp the line right below the capping station to allow you to run a nozzle wash. obv. making sure to remove it before the nozzle wash cycle is over.
however, back to the potential of a broken pump/waste line:
If you can access the pump, you could try removing it from the gear and rotating a couple of degrees and re-installing. Step&repeat until the issue is resolved.
noted: you do not need to power-down the machine to do this. Opening the back-door of the printer to observe the ink pump in action is no big deal.
Use this time to possibly observe the pump as it operates, looking for any leaks/cracks/splits in the line near/within the pump mechanism. If it is cracked, you have to replace the whole pump... even if you could remove the old line from it, re-insertion of the new line is such a PITA that you may as well just get a whole new <$100 setup.