I spent many hours yesterday trying to run my fish tape in underground 3/4" conduit that contains 5-#12 wires. It is about a 50' run and it seems to stop in about the same place. The farthest I got was about 30'. I tried connecting the tape to the ground wire and pulling that but that only got about 6" in either direction. I was just wondering what might be causing this to be so difficult? Is there anything I can do instead of disconnecting the wires and pulling everything out and starting over.

Pulling in wires in a conduit already containing wires is often difficult, as the existing wires get twisted and offer an obstruction, often at elbows or sweeps, during the original installation.

Plus if you used a metal fish tape, it can damage the insulation on the existing wires.

Whenever I need to add wires to an existing conduit, I always figure on pulling out the existing wires and add the new to the harness.

Pull out the existing wires, and inspect the entire length of each wire carefully for damage before re-installing with the new wires. If there is any doubt, install all new wire. 250 foot of #12 is not that expensive, and will insure there will be no problems later.

Pulling all the wires is the only way to go. It is faster, will allow inspection of the wires and make the installation easier. (and you will find out what is blocking you at 30')

I like to use 1 existing wire to pull in additional wires. If I pull out a green wire, I will pull in a green with new wires to replace.
Since it only moves about 6 inches either direction, it may have a junction.
Worse case pull them all out, and attach a fish or wires to the end.
Lube Well.
If the wire breaks, it may be from a junction box connection, and the length of the wire will help locate the box.