I'll try a backwash when I get home today - good idea.

As far as the main drain, I guess that's what I was getting at in my original post - I don't know HOW to get the main drain working at all. I don't have anything in the plumbing I can see that lets me switch between the skimmer and the main drain (can you tell I'm still a newbie? ).

There does seem to be an additional "hole" or opening in the skimmer itself - I've always wondered where that led, but it looks like it's plugged up and I don't ever feel any flow/pressure there. Could that be sucking from the main drain and then piping it into the skimmer?