Here's what I've done in the past. Not sure if the individual elements work, but the combination has worked and got pretty much everything recrawled and ranks / traffic restored in a few days ( about 30K pages ).
- Test robots.txt again from the Webmaster central tool. This causes a refresh of what Google knows as your latest robots.txt
- Submit the sitemaps again into Google and other search engines.
- Increase the crawl speed.
Like I said, these three were done and worked for me in Feb. Not sure if one individual step is enough.
And needless to say, this all depends on how well your site usually gets crawled.
One more thing... we do not have individual pages sending out if-modified-since headers so not sure if this is also a factor in getting recrawled / ranked.