recurring tempeh problem

[Extremelylara]

Can anyone diagnose what's happening with this tempeh? The problem is NOT due to heat, incorrect starter quantities, bag perforations or vinegar ratio otherwise it would have effected the whole batch. The problem occurs only on the blocks that were from a certain vessel - the rest of the batch (from other vessels) turn out perfect. Each vessel contains the cooked, spun beans which are weighed, left to cool, vinegar added, starter added, mixed well, then they're bagged up. The vessels are large plastic bowls (cleaned and sterilised). In this latest case, one vessel produced tempeh like this (dark parts are soft, smell slightly of ammonia, and taste bad). Whatever is happening, it's happening in the vessel at the mixing/cooling stage. My best guess is bacillus? It happens occasionally over the 7 years I've been making tempeh on a large scale (commercial producer) and I can't work out why.