Exposures all over the place?? HELP!!

Hi there,

So I usally shoot on location (outside) with only a speedlight on my D200. Recently I got two stobelights with softboxes - right now I am just using my speedlight to trigger them.

Here is the problem... indoors in a space with no changing lights ( no windows etc) I set up my two softboxes and speedlight (on camera) - set my D200 to the settings I metered for. Start shooting - holy crap every exposure is diffrent .. they are all over the place.... some are exposed well .. some are ok.. some are black. I havent touched any buttons between shots?? What is going on?? Is there some basic piece of knowledge about flash I am missing.. I have read alot but cant find anything / anyone to walk me through this!! I am doing portraits by the way. If anyone can help it would be GREAT!!!!

Cheers,

Kristi