Problems with shares

[Gieter]

Morning people,

I've been trying to mine some today for the first time. I got 3 machines running, 2 slower machines and 1 fast machine.

For some reason the slower machines are getting shares as they should, but the faster machine is not getting any shares in over 7 hours with 2 times 300 m/hash.

Am i doing something wrong or what could be the problem with my settings.

Im am using GUI miner.

Hope someone has an anwser for me asap.

Thanks.

[Gieter]

Some more information:
2013-04-19 03:10:00: Listener for "GPU1": 19/04/2013 03:10:00, started OpenCL miner on platform 0, device 0 (Cypress)
2013-04-19 03:10:00: Listener for "GPU1": pit.deepbit.net:8332 19/04/2013 03:10:00, checking for stratum...
2013-04-19 03:10:01: Listener for "GPU1": pit.deepbit.net:8332 19/04/2013 03:10:01, using JSON-RPC (no stratum header)
2013-04-19 03:10:16: Listener for "GPU1": pit.deepbit.net:8332 19/04/2013 03:10:16, Verification failed, check hardware! (0:0:Cypress, 9659a969)
2013-04-19 03:11:01: Listener for "GPU1": pit.deepbit.net:8332 19/04/2013 03:11:01, warning: job finished, 0:0:Cypress is idle
2013-04-19 03:11:06: Listener for "GPU1": pit.deepbit.net:8332 19/04/2013 03:11:06, IO errors - 1, tolerance 2
2013-04-19 03:11:06: Listener for "GPU1": pit.deepbit.net:8332 19/04/2013 03:11:06, long poll IO error