It's hard to guess why it might not be working for you without looking at a capx to see if it's something obvious (not that I can think of why that might be). Are you tinkering with the config.xml file at all?
If you are an amateur like me then I suggest having a good read through the phonegap build documentation and I posted some settings I found in other forums on this thread which might be of help. If your game has a lot of graphics or demanding plugins then it's possible that phonegap simply can't cope.
Post a capx if you get stuck - I can't think of anything else to suggest.