The instructions for timeago suggest that you need to use ISO 8601 format for your timestamps:
<abbr class="timeago" title="2008-07-17T09:24:17Z">July 17, 2008</abbr>
<time class="timeago" datetime="2008-07-17T09:24:17Z">July 17, 2008</time>
Also from the instructions:
Are you concerned about time zone support? Don't be. Timeago handles this too. As long as your timestamps are in ISO 8601 format and include a full time zone designator (±hhmm), everything should work out of the box regardless of the time zone that your visitors live in.
I find it odd that it is working for you otherwise in some cases, but you should probably change your timestamp format and see if that resolves your problem.