i had this problem with my MP7 and although this youtube video is kind of amateur this is what helped me figure out what was being said in the OP so i figured i would post it to help anyone else having this problem
the part you need to see with the explanation is at 8:30 into the vid (part 1) and he describes how to correctly position the sector gear and check when reassembling.
seeing this with the description made it all click for me so if youre having this problem hopefully this clarifies it for you too!
YouTube - Tokyo Marui MP7A1 Gearbox Part 1