That sure sounds like it could be the problem. I’ll keep this in mind for next time. Thanks!
Yes, that is 99% likely the problem. This is an issue I deal with regularly as an audio engineer. Depending upon the encoder your engineer is using, the default is most likely VBR, and s/he either needs to de-select VBR (Pro Tools) or actively choose CBR (Wavelab/Myriad/etc).