Memory Problem with parallel computing toolbox

Hello, I was wondering why... and I would appreciate if someone could answer to the following question.

As I leaned about the parallel computing toolbox with 'workers', I tried to run several processing (fMRI analysis using SPM) at the same time using several workers in one Matlab window... but! it was stuck and frozen and... I couldn't do anything about it. Next time, I opened 4 separate Matlab windows in one computer, and then run the analysis in each Matlab window. Then, there was no problem and all the processing was running fine.

My question is... why one Matlab with several workers wouldn't work (frozen), but several Matlab windows with one worker would work fine?

Thank you so much for your advice in advance!

Best, Naho