Cordoba F7 Paco Flamenco Iberia

After 4 month of usage , it started to buzz in string 6 ,5 and 3. problem reported to Thomann and sent it back. They said they didn’t find any problem and offered to pay me back minus 10%.
So much for 3 years guarantee.
Why shall I receive 10% less if you can not fix or even find the problem.
The after sales technician who tests the guitars are not professional guitar player and can not play different techniques,that is why can not find the problem.