I'm unly running a very small rig (6950 = 250 Mhashes/s).

On BTCGuild I got around 0.088 BTC per day.
According to mining-calculators I should get at least 0.14 BTC at the current difficulty. Any ideas what's going wrong?

That step is pretty important because at a price of 22 euro-cent per kw/h mining is already not profitable anymore at 0.088 BTC / day.