How strange! Millions of people have been successfully using "beurriers bretons" for centuries. Why would it be so different with you? :-/
Did you change the water every 2 days, as recommended, or change it daily if your environment is warm (using fresh cold water, of course)? You do have a clean water source, do you not?
When using this ingenious pottery correctly, butter will easily keep well for at least 20 days' if you follow instructions.
Product feature erratum:
"Butter Keeper prevents butter from melting' SHOULD BE REPLACED BY: "Butter Keeper prevents butter from melting and spoiling.'