On 22 February 1936 thirty AMR 35s had been produced; twenty more were being worked on. By the end of June 1936 production numbers had reached 76 tanks, 41 of which had been delivered. At that moment however, a very serious reliability problem emerged that was to slow down production considerably. Many mechanical parts proved to be too weak; especially the Cleveland differentials were not capable of taking the stress loads caused by the high speed of the tank. Much of the factory capacity in 1937 was used to rebuild and modify the entire first production batch of 92. On 8 April 1937 Renault was forced to submit two modified vehicles — one of each of the two main versions — for acceptance as "prototypes". This upgrading project did not result in a very reliable vehicle though: in October the Ministry of War was amazed to find out that out of 43 vehicles, of which the differential had already been replaced five times, six were again in need of a new one.