Now we're not in the Whine & Cheese forum any more I can't say "Your ****ing problem is your ****ing cookies you ****ing ****head!" Just a moment to shift gears here....
Okay -- your problem is in your cookies. See, if you have "remember password" checked then Bladeforums recognizes you by cookies, which are two text files stored on your computer. They get changed if you change your options, between times they just sit there and your browser sends them when Bladeforums asks for them, and that's how Bladeforums knows who you are and how you want the time and date to be shown and things like that. If anything happens to those cookies Bladeforums doesn't recognize you, so it says you're not logged in.
If you have your browser set not to allow cookies it won't work. If your browser crashes at the moment it's accessing the cookies the files will be corrupted and it won't work. If you click on the "delete all cookies" button or if you delete the entire directory your browser keeps its temporary files in (cookies are considered temporary files) it won't work. Sometimes for no discernable reason it won't work because computers are inscrutable.
If it only happens once in a while it's probably either because your browser crashed or because computers are inscrutable. Neither of those can be avoided at present; browsers won't become crashproof any time soon and computers won't become scrutable in the near future either.
The way the current Bladeforums software works, if anything goes wrong when you try to post -- you aren't logged in, the server is overloaded, you try to post an attachment that's too big, the Net Demons take a bite out of your packets -- if anything at all goes wrong, your post is lost. If you try to go back to it you get "that page has expired." That sux, and I expect the vBulletin people will fix it sometime -- don't know when, but long before browsers stop crashing and computers become scrutable.
In the meantime I can only suggest you write anything longer than a couple of lines in your favorite word processor and paste it in. If you don't have a favorite already Wordpad is good enough for most people and that comes with Windows. Create a file called posts.doc and write all your posts in that and save it and back it up frequently, and paste in your posts from that, and you'll never lose another post.
That has several other advantages -- if you use a real word processor (not Wordpad) you can do spellcheck and you can customize it to suit your taste, use macros, remap the keyboard, etc. Besides you'll have a permanent copy on your own computer of everything you post, and who knows, you might want it again someday.
For instance, somebody else is bound to ask the same question, and when they do I can paste in this same reply and won't have to write it all over again.
