I had this problem as well- while it seemed that the caulk there was lacking, that didn't work. We ended up caulking BEHIND the corner of the slide where it attaches to the "box". i had lots of gaps there as well, and we figured it followed the track in. I don't have pics now, but take a look behind the wall when it is extended and caulk what you see. We used dicor for it, really got about 1/4 of a tube in there. This has worked for us.
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