How can I remove a stuck coax cable from a conduit?
Unless it is extremely thick co-ax, like RG-35, there should be plenty of room to slide the nylon fish tape from outside along the stuck co-ax.  Probably the wire you wrapped around it for attaching to the fish has formed a barb&mdash;how apropos for a fish!  :-)

Push the fish past the end of the co-ax a little bit (or as far in as it will go), and then alternately pull the fish back and then jab it forward in a series of vigorous thrusts intended to make it slide past the end in a random new location each thrust and, if it hits the target, bend the barb out of the way.

If that doesn't do it, get some silicone lubricant or any other non-drying lubricant, blob it liberally on the end of the fish and push it in to the end of the co-ax in an effort to lubricate the area, twist it around a bit, and then withdraw the fish and repeat 6+ times to be sure there is lots of lube in the conduit.  Remove the fish when done.

If the conduit where the obstruction exists is surrounded by concrete, I would try giving the co-ax a sharp, swift, hard pull to see if it can be budged into the lube.  If it is not so well supported, then pull gently at first and gradually build up force allowing the co-ax to stretch:  if it can be made a little thinner, it might let go.

Give that a go.  I have some more half-baked ideas if that doesn't work.