How do you fix gel pens which quit working, but have plenty of ink in them?
If you are sure the ink has dried up at the end, then soak it with a solvent.
eg cleaning alcohol.
If you think the air bubble is there, you can try to tap or flick the ink down,
centrifuge it (which is another means to use inertia to create the force to drive the ink down.)
Or you can try to boost the air pressure up at the other end… creating a pressure differential… force… lung pressure can be enough…
I use purple G2’s and like most colors of pen other than black or blue, the cartridges are… less than reliable.
First, I keep more than one at hand.
Second, I take the cart out (when I have a moment) and give it a gentle squeeze while writing hard. Works pretty much every time and I really, really do have some that are down to their last few millimetres of ink. I have pics!