Well, 300kh/s isn't much for a set difficulty of 512, but you should find some nonetheless (about every 2 mins or so).
Are you seeing an accepted hashrate on the server for your rig when you look at the webpage? If am not mistaken, when a coinswitch is executed, and you are still busy on the last one, cgminer will think it's a stale and not show it as accepted, yet the server will.
Can you pm me your config or .bat settings and hardware (pls don't post them here there is enough clutter in here already).