When I tried to submit an entry, it gets a "Failed" status.
I already followed the format when compressing the 2 txt files (also updated the decimal places to 5 for the predicted probabilities as per the sample submission file).
Or does the leaderboard only ranks the top15 and omits the rest (the reason for the "Failed" status) ?
I already resolved this by renaming the compressed file to "submission.zip".
None of the reasons you've listed is an issue. The decimal places can be more or less than 5. The name of the zip file is not important either.
After investigation, I find that the issue is the names of the prediction files. They should be named: "clarity_valid.predict" and "conciseness_valid.predict" (exactly like those without further file extensions).
In your current submission, the two prediction files have ".txt" extension. Please correct the file naming and submit again.