Are you using your iPad in the same location every time? I don't know if it matters but sometimes when I travel websites don't recognize my computer... since it's away from it's home IP address.
Another problem could be that you have "private browsing" turned on. If this is the case, you're telling your computer to forget whatever cookies it picks up from the websites you visit. These cookies are little packets of information that the computer uses to tell the website "I've been here before! Look, this is what we did last time!"