There’s a couple reasons why this might happen.

1. The artwork size is too large. We cap the file size limit at 25mb so please check what the size of the file you’re trying to upload is.
2. The dimensions of the artwork are too large. The max dimensions we allow are 10,000 x 10,000 pixels. Anything above that will not upload
3. There might be a caching issue causing the upload issues. Clear the cache on your browser. See instructions on that here