A recent study tried to explain individual boar semen quality variability responding to cooling time and extender type.
The overall results of this study provided evidence that there are differences among boars in response to the different cooling regimens. The authors pointed to improve the technology of boar semen cryopreservation.
There were great variations in progressive motility and the sperm motion patterns before freezing among the boars which become even less predictable after handling and processing semen.
"If you can't measure it, you can't manage it". Check semen quality whenever you suspect any environmental effect before artificial insemination.