Many FTP sites throughout the world hold the whole Project Gutenberg archive of texts. An FTP site is just a computer on the Internet that specializes in holding files for download and sending them to people on request. You can find a list of FTP sites that hold Gutenberg texts at <http://www.gutenberg.net/list>.
When you're searching or browsing for titles and authors, you're on this Project Gutenberg site, but if you choose one of the mirrors, or another method of downloading, when you click on the book to download it, you are connected to an FTP (or HTTP) site. At the time you click on the filename, your browser contacts an FTP site and tries to download the file from there. If you get an error, it could be because the FTP site is busy, or because there's a network traffic bottleneck between you and that FTP site, or because the text you're looking for is missing from that FTP site.
Usually, the easiest solution is to choose another FTP site to download your text from. Go to the Search page, choose a different FTP site, and search again for your text.
Tip: You should always try to choose the FTP site closest to you. Not only are you helping to minimize Net traffic by choosing a nearby site, but your file will download faster!
If all else fails, note the year and the filename of the book you want, if it's below number 10,000, or its number, if above 10,000, choose an FTP site from this list and click on one of them. Then browse your way through the listings to the file you want.
For example, if you find "Lady Susan" by Jane Austen, you will see that it was published by Gutenberg in 1997, and its filename is lsusn10.txt, so browse to one of the FTP sites, choose the directory called /etext97 and click (or right-click and Save, depending on your browser) on the file lsusn10.txt. Or, in the case of Clarissa, Volume 6 by Richardson, which is #11364, you will find it in the directory /1/1/3/6/11364