Caustic Soda Etch Tank Suddenly Causing Intergranular Attack

Q. We have a chemical etch tank that is made up with Oakite 160 (NaOH) and used exclusively for etching aluminum. The tank's maintenance includes routine 49% dumps. Every three months, we process test panels that are made up from 2024 bare aluminum and have them inspected for intergranular attack and end grain pitting. After 8 years of maintaining this chemical processing line, we have never had any intergranular attack detected until now. The test specimens we processed in the tank are provided to us by the company that performs the intergranular attack and end grain pitting inspection and tests were conducted with material from to separate lots, so we don't believe the reason for the failure is due to the material. Any suggestions as to a possible reason for the sudden intergranular attack would be appreciated.