If you get a sale or lead and then it suddenly dissapears then that normally means that it was an invalid sale or lead. In other words the customer may have had a refund or the sale / lead was a fraudulent one, etc. If you know that there were a sale or lead made and it was not tracked at all, then I would contact them to see why as it could be a problem. Sometimes the affiliate network may not realise as it is also up to the merchants theirselves to also make sure that their pages are set up correctly so that the networks can track the sales and leads made from their site. Some merchants change their site and puts the affiliate networks tracking codes in their pages wrong and then sales do not get tracked. Things like this seems to happen to often with the larger networks like CJ, LinkShare, ShareAsale, the UK networks, etc.
Yes, that's correct. Often affiliates "test" a campaign to see if tracking works but don't see a lead and think something is broken. 9 times out of 10, nothing is broken, the test sign up just didn't generate a valid lead. (didn't confirm an email address, used junk sign data, bad IP, didn't select a coreg, email already in the merchant database, etc, etc). While a sale is pretty obvious, most merchants define a valid lead differently and often it's not very straightforward. That said, tech problems do happen with all networks. Pixel tracking isn't 100% accurate as some affiliates surf with their cookies turned off.
Another thing is that I am not 100% sure on this, but I think some networks will track a sale, but not list it as a sale in your stats untill it has actually been confirmed properly by the merchant.
You are correct. Not all sales are reported in real time as well. So sometimes there is a delay between a click and reported sales. In addition, sometimes a sale is not a sale until confirmed. For example if someone places an order and chooses to pay by check, the transactions is usually not counted as sale until the check is received and clears the bank -- so you might have 1-2 week delay because of this.
It could be a technical problem.. I had that issue with Yahoo Search Marketing's CJ affiliate program - the link that I was using stopped tracking leads for a while. I never got credited for those sales, unfortunately. The only way I found out about it was when a client used my link to sign up for an account, but it didn't actually appear as a sale in my account.