The tracks are color coded. When you import your midi, is it all one color? If it is, i would recommend going into an app like signal.vercel.app, then try removing some extraneous tracks if there are any and then update the channels to be just in the first four (since the app separates just the first four channels, then sets the rest to one channel).
Generating off of a certain length would be a good idea, when i'm able to create larger context models (For now though, it is probably not worth it because the current ai models really needs the context)