Currently working on a project that has 1 Cakewalk TTS-1, 1 Cakewalk Soundcenter and 2 of the same 3rd party VST loaded onto instrument tracks. I would now like to load up another track of the 3rd party VST but all of my soft synths are now greyed out, including the Cakewalk ones. It seems to be just this project - if I open another they are available for use.
 
Any ideas guys?

Open the Synth Rack. Delete any synths you aren't using.

Yep, that's done the trick.
 
Thanks Greg, much appreciated.

Cool.
Closing a synth's GUI by clicking the red X doesn't remove it from the project. Each synth has to be deleted from the rack, and MC has a limit on the number of synths you can use simultaneously.