The JX-8P uses a compressible and conductive rubber between two metal films to measure the aftertouch pressure. With the time, the metal films oxidate and the aftertouch effect disappears. By cleaning the metal films you can successfully get aftertouch back!
Roland fixed it during the production (service info 100258, "JX-8P After Touch Problem").
Following the instructions of IOTech and the pictures of StirliTZ, I've successfully restored aftertouch!