Salt content?  Is it possible we have a natural variation in salts 
produced in our milk, even through the course of a pumping session? 
It might be interesting to experiment with the milk:  if mixed with 
several freezing-doses (preferably from the same pumping session), 
will it then freeze (is there something 'dominantly' "infective" 
about the non-freezing milk or is it "dilutional"?)  Any chance such 
a salts-fluctuation could be related to mom's hydration?  It's 
conceivable the issue reflects some underlying medical condition in 
mom and could be of some use to cess out?  Someone with a 
spectrometer of some sort -- not sure how or what would be tested for 
molecularly.  Also - is it in different bottles that it sometimes 
freezes and sometimes doesn't?  Mark the bottle/s with non-freezing 
samples.  If some salt (e.g.) were leaching from a specific bottle 
that would be terribly important to know.  Or as simple as a relative 
hot-spot in the freezer; presumably you've tried just moving the 
offending sample?

Interesting!

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