I have this problem off and on. I don't know if you are aware that LJ has a recent comments tool (http://www.livejournal.com/tools/recent_comments.bml) that gives you the last 50 comments posted to your own LJ -- although not any responses to comments you posted anywhere else. It can be a handy way to check and see if things are getting missed.
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I'm also using gmail, but having had them route any LJ stuff to spam yet, fwiw.
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