are you getting yellow ink to the heads? if you pull ink into the damper with a syringe, then pull that ink through the heads below (pump line or waste line) does the ink come through? if not you may not be getting a seal at the yellow cap. jv3's have a cammed capping station that work decent but not as well as a mechanical/vertical raising capping station.
on this printer, the capping station often needs to be shimmed on the far right (yellow). i used to have two razor blades set under my capping station on the right.
if no yellow is coming through when drawing at the yellow line below the caps,
have a partner push up on the capping station (upwards). then pull again. if you get ink you can blame the angle of attack (there are three settings in the firmware that set the carriage position relative to the capping station), the height (on the right), or the springs in the capping station (you can stretch them out a bit for more pressure), and of course dry/warn/cracked up caps.
also there can be a clog right below the cap as the tube connects to the cap and a clog in the pump. i doubt this is the issue. less common issues can be a cracked head manifold, bad o-rings, loose fittings.