Sorry for the delayed response. Sometimes when you try to upload via cellular data you may run into problems, especially if it's a large file. I am also suprised to hear that when uploading via Wi-Fi you were having problems. Have you tried un-installing and re-installing the Dropbox App and/or clearing your browsing cookies & cache on your phone? If no, please do so. If none of the following works, please reply back and I'll do my best to help you. I hope this helps!
Did this post help you? If so please mark it for some Kudos below.
Did this post fix your issue/answer your question? If so please press the 'Accept as Solution' button to help others find it.
Did this post not resolve your issue? If so please give some more information so we can try and help - please remember we cannot see over your shoulder so be as descriptive as possible!