Hello Christophe,
First, review the following help topics on how the comparison works and why it can fail:
Factors Affecting Image ComparisonWhy Image Comparison Fails.
Then, try tuning comparison options so that pictures that differ only in few pixels will be recognized as the same. Review the
Comparing and Finding Images - Specific Tasks help topic in this relation.
If the comparison still fails for some pictures, please send us the entire project suite with results of the test that fails along with the pictures to compare. We will try to find the exact reason why the recognition fails.
