There could be many things going wrong in this situation, so it is difficult to give exact advice.
This could be a settings issue on the 1) Hikvision side or on the 2) Dropbox side, or a 3) connectivity issue.
I would recommend starting with your Hikvision setup, making sure you haven't missed any important steps.
You might also want to check that your Dropbox is synced, see also the camera upload article which may offer you some troubleshooting options.
Best of luck!