Just a guess, but Groovymame may be confused about what your monitor can do.
In Vmmaker, last tab "mame", browse to find your Groovymame executable and tick the box "Export settings to Groovymame".
Then press OK and you will see a message in the command window "
Exporting settings to mame.ini...", which is fairly self-explanatory.
Must be a bad echo in here? I keep hearing my own voice saying that same thing
Like most of us, Calamity is a busy guy, and will normally not provide any more meaningful comment unless you provide a log file
Instructions on how to do that are in his signature line, bottom of every post.