How can a transistor lose its gain?
There could be different causes depending on the type and construction of  transistor, and any environmental stresses it has been exposed to. These may include diffusion of atoms in the semiconductor, electromigration of the metalization, contamination by water or oxygen, excessive voltage or current and mechanical or thermal stress.  

I could not find any information relating to this failure mode in germanium transistors, but I did find an article on the degradation of silicon transistors, which says this:-


  The current gain of bipolar transistors strongly decreases when
  the oxide over the emitter-base junction is damaged. This can
  occur in normal operation conditions since, when the emitter-base
  junction is reverse-biased, hot carriers are created...
  
  the base-collector junction is not degraded by 
  the stress.


The accompanying graphs show dramatic gain reduction,

  

but relatively normal Base-Emitter diode action.



It may be that germanium transistors are now getting old enough to show signs of degradation during normal use, however the failures of this type that I have come across were all silicon transistors that were stressed by over-voltage.