Help with extended desktop

I have a 21.5 iMac with a 25" thunderbolt display attached. When using internet, applications etc.. I have no problem with the extended desktop.

When i open a remote desktop through a citrix connection however it won't extend the screen onto the Thunderbolt. It will allow me to mirror, but not extend.

Any ideas how I can resolve this? When run on a standard windows Laptop and monitor i can extend the desktop easily