Take a look at the Log file (from the Application Manager pull-down menu) and you will see which repositories have problems.

If you've "pulled" all of those from gronmayer's website, you will have a lot of errors reported. Of the 50 repositories listed, I only have 39 enabled as a result of all of the errors (checksums mainly).

Also, there are occassional problems with a repository not responding within the timeout time (due to server load?) and it will report a failure.