IRC is awesome, but it requires a lot of work. One of the reasons is that there are often dead bots that still list but fail to transfer for any multitude of reasons. You can try disabling your firewall, particularly if it happens close to 100% of the time, but it is likely just a dead bot. If you see a file in great demand with most slots or all slots open, that's a good way to tell even before you try.

I'm sure there are several, but nothing really useful as far as the xdcc feature is concerned. You may want to look into using fserves; they are often slower, but more reliable, as they are set up willingly by people for your perusal instead of being a hijacked comp open to all sorts of unknown variables. They require a little more digging and prying, as they often don't list the files in a channel and use dos commands to navigate the files, but if you can find what you need, you *usually* will receive the file pretty stably to 100%, albeit at a slower rate on average than most xdcc bots.