Do these older copies contain "conflicted copy" in their filenames? Stash is built to deal with conflicts by modifying the names of the affected files and allowing for multiple copies.
Can you send us the log files from both computers? Send them to stashbeta-support@mozy.com and I can have a look. You can collect the log files together by right-clicking on the Stash icon in your systray, then select Help, then Collect Log Files... Let us know in your email what file was involved in the conflict.