Basic newbie question.

[Steve Newell]

Hello Everyone,
I have got myself a Roland EGX-400 with Vcarve 8 just doing basic machine tags onto rigid laminate.
My question is: Sometimes, once I have highlighted the text, selected quick engrave, selected tool etc. When I hit 'calculate' nothing happens. But this is intermittent. Sometimes it does calculate the toolpath, sometimes it doesn't. I don't think I'm doing anything different each time. Any help would be appreciated.
Steve