It might be the case that the audiences you uploaded contains 8000 members, but less than 500 of these members were active in the last 30 days and thus the audience is still deemed too small.
It’s also possible that the data you’re providing is not correctly normalized and hashed, and thus the Twitter system can find no matches at all. I’d encourage you to take a look at our Tailored Audiences File Data page and ensure that your lists are correctly formatted. We also have a hashing code sample on Github that might be useful.