Some of the possible causes of this problem:
1) Where did you purchase your computers? Were they "off the shelf" or were they from a computer store that builds their own computers?
2) What are the hibernate and sleep settings on your computers?
3) What anti-virus software do you have installed on your computers?
4) Sometimes there are programs on computers that continually search for "available" USB slots. They will take slots determined to be available even if they are technically in use. Are you aware of any of these being on your computers?
5) Are the computers networked?