this is because the "groove" that is cut in, isn't that deep, so the clip isn't that thick.
remove the 3rd/4th counter shaft, take to a machine shop, get them to deepen the cut for the clip, then get a bigger, stronger circlip to go in the new deeper cut groove!
I picked up a slightly damaged S15 box for $80US, yes $80US. The guy was frustrated and put in an S14 5 speed. I am gonna take my time and strip it down and hopefully it's just the circlip that's the problem. Will get some pics as well.