maybe there is a language issue here - if they are available on line, what is the problem? did you actually download them? the history folder just contains URLs, not the web page it self. that winds up in a local cache. just because the history entry is there doesn't mean the file still is. The term TTL means time to live and pages can have a very short TTL. once that has elapsed, your browser has to go get a new copy.
Or you could just be flushing your local cache. maybe you are running out of disk space and your internet cache is getting purged automatically.