It's the water wieghing down the string. Using a 5" proper sized mouse might help, but with water, it will sponge up enough to also weigh it down too. Sounds like you need a 460'+ fish tape, (You can get them in 500') or wait until the water is gone....
Blow the compessor for a few hours through the conduit to help get some of the water out. (Not that it'll do much) Then put a 5" mouse on a 500' fish tape, and help the compressor by pushing it in too, is your best bet. If it gets stuck, pull it out quick, and hopefully a bunch of water will come out with it. And wrench out your mouse... You might have to do it a bunch of times to get past the water.
