I am having the same issue as the original poster. When I try to search for metedata, I nearly always receive an error message: "ERROR: Could not find metadata: The metadata download seems to have stalled. Try again later." Sometimes the search will be more likely to complete if I find the book's ISBN elsewhere and fill it in before searching, but even this doesn't always work. When I try to download metadata, on the rare occasions I'm able to get a search result, the download usually doesn't complete--no error message, it just keeps me waiting without a result until I cancel it. Upon hitting cancel, some of the fields sometimes do populate with downloaded metadata, but usually not in full.
Metadata searches and downloads used to work flawlessly for me, until these problems began within the past few months/Calibre versions. I don't believe I've changed anything on my system during that time that should have had any effect on the issue.
OS: Windows 7 Home Premium 64-bit, SP 1
Metadata download plugins enabled: Amazon 1.0.0, Goodreads Metadata 1.2.0, Google Books 1.0.0, IsbnDB 1.0.0 with access key (I've tried enabling and disabling many plugin combinations, with no improvement)
Please let me know if I can provide other information that may be of help.