We solved the problem with the person responsible for the NMR.
First, there was a problem of shimming, the z5 and z6 parameters were out of the limit with theses compounds. By doing it manually we solved the problem.
Then, we started to heat the sample with the following temperatures:
-20°C, 40°C, 60°C, 80°C, 100°C
100°C happens to be the best temperature the signal is well improved ! I assume tautomeric forms are the explanation.
Thank you for all your help !