Do a full update on the group that contained those items.
If you want to post a comment, thank-you note or a request, you can use the post-function in Sponge. If you want to post files, use powerpost.
Select the Server option on the Block button sub menu and then click on Block. The server used by the poster will be automatically added to the filter list.
The most popular groups are also the groups that will take up a lot of bandwidth, especially if a lot of people use it. This is why the most popular groups aren't carried by some newsservers. If you want to get these groups, you'll have to use another server which will cost you some money.
Something went wrong during uploading. First of all try redownloading the corrupt file(s) several times. If the redownloaded files are also corrupt, something went wrong during upload. Contact the uploader and ask for fills, do NOT start flaming him!