Interesting frozen semen problem

This question came from an A.I. Centre in India.

3 Gir bulls, Semen Processing Centre in India. Collected together, processed together (Andromed), same LN container. Same canister?

Problem in post thawing motility in one of these bulls, only.

Post-thaw motility (PTM) of sperm similar in all bulls immediately following freezing and up to 1 week later. Then, gradually decreases, viz

PTM

 – after freezing, 50 %
– > one month, 35-40 %

– > 2 months, 20-25 %
– > 3 months, 10-15 %

– > 4-5 months, 0 %

PTM does not change over this time in the other 2 Gir bulls.

Suggestions, solutions?