Duolingo frequently cuts me off half way through entering my answer and marks me wrong.
Try putting this in the troubleshooting forum with more detail (are you using the app or the website, is it lessons or practice sessions, what browser are you using, etc.)
I was having trouble with that when I discovered that instead of hitting the quote mark (for accented characters using the International keyboard), I was often hitting the Enter key. (I still do that sometimes in timed practice when I get behind and try to type too fast ...)
I know that it's not likely you're having the same problem, but just in case ...