I too was having troubles with my Hi-Hat on my DTX700 kit recording into MIDI with Garageband. I am not exactly the most savvy on speaking MIDI language but I will try my best. Here was the problem/how I resolved it:
I had a problem where when I used the preset drum kits provided by garage band, the edge of my hi-hat would not make sound in recording/playback. The top of the hi-hat and all other aspects registered fine but not having access to the edge of a hi-hat is pretty frustrating! Upon further assessment, the notes for the edge of the hi-hat were being recorded into Garageband onto the MIDI track but were going onto a note that does not have a sound designated to it.
I resolved this issue by using MidiPipe to re-designate (remap) where the note was going. I remapped the edge open hi-hat note to the same as the middle open hi-hat note where I was getting sound. Then I did the same for the edge closed hi-hat to the middle closed hi-hat to have same sound.
Refer to this video on how to do so:
How To Remap MIDI in Garageband
Also, if remapping more than 1 note, do the inputs/piping like this on MidiPipe:
Hope this helps and isn't too confusing. Use the video though. Helped me a lot!