My dual flush toilet wont fill up?

I have a fairly new dual flush toilet but recently the cistern doesnt fill up correctly. I have managed to get the lid off and takea look inside and water is coming in through the small valve at the top of the left hand mechanism (sorry i am not familiar with any of the terms) This valve seems to be fitted with a small arm which closes the valve when the water is at its set level. The other end of this arm if fork shaped and seems to slot around a rod which has the float attached to it. I have managed to get it working once and noticed that waer should fill up from the bottom of this mechanism until the float reaches its correct level and it then closes the valve at the top and all is fine. However as i said this doesnt work and no water is filling up the tank, instead it is just squirting through the valve at the top and is not cutting off when the float reaches its highest point. Water is then going down the overflow Can somone give me an idfea of what the problem could be and how to resolve it.
Thanks Craig