You can increase the gel concentration to 15%, it can resolve smaller proteins between 15-25 KDa. Also, make sure all the preparations are fresh, especially APS and TEMED should be freshly added.
i do not thin k that there is something wrong, the problem is that for so small MW you have to use higher acrilammide % (e.g 17% or 4-20% gradient). Probably you can try also tr reduce the running length with your current % but it will result in a poor separation at high MW.