If your upload is smaller than 8GB you don't even need to create a torrent, you can hash it directly from the upload page.

Then compare what's actually in the folder against the list shown in uTorrent. See if you can spot a small file that is shown in uTorrent but not in your file directory. Some "padding" files, perhaps, which are usually pretty small? Nowadays, these are usually unnecessary but there's often a setting to specifically ensure these aren't created.

The files need to exactly match. If you have, perhaps, opened an image file and trimmed it slightly, or maybe even simply amended the file's properties, they may well not.

I'm sure these are basics you've already thought about, but thought I'd mention them just in case not.

Once you've created the torrent from a directory, all those files need to stay in that directory, unchanged, to be able to seed it. You can, though, add additional files to that directory. That does does no harm. The torrent just wants to know "What directory is it?" and "Are these files here?". It cares not one jot if there are also other files.