Sometimes the problem of durability is not clearly defined and the statement that ‘the adhesive is failing’ is not particularly helpful but ‘5% of the parts fail after 6 weeks in a hot, humid country’ is far more useful information to the engineer.
A similar question that might be relevant is: ‘Have the failures occurred with this batch of adhesive/components or do the failures occur on multiple batches?’ It is these kinds of questions that will help to identify the failure.
One technique is to try and replicate the failure mode. If the failures can be replicated under controlled conditions then it is likely that a better understanding of the failure mode can be obtained.
The lack of performance in an adhesive joint could be due to a number or a combination of factors although the most common environmental problem for adhesive joints is water or high temperature with high humidity.