Here's s few items to check as part of your trouble-shooting.
- Is anything amiss in your robots.txt?
- Is there an accidental "noindex" left over from a test version of the page?
- Does the page have any coding / validation errors?
Validation errors don't usually affect ranking (so far as we know) but once in a while the wrong error can stop a spider in its tracks, so when a page is having problems it's worth checking how well it validates.
Don't ask how I know about these things ...