Hello,
Any chance you have Dropped Call Recovery enabled for the affected subscribers?
I found the text below in the System Admin Guide: http://www.cisco.com/en/US/docs/voice_ip_comm/unity/5x/administration/guide/ex/5xcusag160e.html#wpmkr1074066
"If the Send Message When Subscriber Hangs Up or Call Is Disconnected option is not enabled, the message will be discarded."
One other possibility is that the recordings are shorter than the minimum length defined for recordings in the SA.
I hope this helps.