is there latent ink in the cap? first diagnose if you are getting a good seal at that cap. not sure of your pump/cap configuration, but, also you need to check if you have a clogged pump to that cap. common clog points are directly beneath the cap, within the cap itself, unions where cap tubes meet pump tubes, inside the pump, at the very tip end of the drain line.
to diagnose if you have a clogged pump:
you can do a simple pump tube swap to see if a different pump line magically cures the issue, if so, you might need to either flush that pump line or replace the tube inside.
it it's not pump/vacuum oriented, then do a data swap on your printer. but check first with someone that is familiar with the layout of your particular machine. data swaps are completely safe if done correctly, and can fry slider boards if done wrong. this would be most suspect if you are seeing ink flow to your cap tops and through the pump but do not get a print. data issues can be damaged data ribbon, bad head, bad board, fuses etc.