First double or triple check everything.
Then try the v0.6 test version if you dont have it in use yet.
Note that everything is case sensitive, if the snippet name is MaxiGallery, you must call it [!MaxiGallery!] and not [!maxigallery!].
See modx error log and possibly php / apache error log for possible error messages.
You could also try to increase your php error reporting level if you are running on a development environment.
If everything else fails, post your exact setup and snippet calls.
"He can have a lollipop any time he wants to. That's what it means to be a programmer."