Do you use the me line follower module in normal way? May I know is there any special application?
Normally, the line following sensors won't break easily.
For the warranty issue, I am sorry to hear about that. Normally, we have official warranty policies and procedures, we won't refuse the warranty for our products which under warranty scope.
May I know where did you buy the robots? Could you send the receipts of the robots which have faulty line follower sensors to the email address (firstname.lastname@example.org)?