Not switching to application window on clicking application

For the last week, i've had this annoying problem, of clicking on an application in my dock the expecting mission control to switch to the space and bring the application window to the top. however this has not been happening, and i'm having to switch to mission control view and then search for the application which is much more difficult than previously when the windows were laid out in a matrix, Does someone know what may be the problem? is there a setting i need to change, have no idea why this suddenly happened.

iMac 27" 3.4 i7, 16GB Ram, 2GB VRam, 256SSD + 1 TB HD running Lion