New iPad won't recognize my router

[origun]

I have had an iPad 2 for about a year and had no trouble setting it up and getting it to communicate with my router--an Airport Express (about 5 years old). I have an old Mac desktop wired to the router and have a laptop (Macbook Pro) and my iPad using it wirelessly. I bought a new iPad as a gift for a friend who is easily frustrated with technology. So when it arrived, I tried to get it set up. It "found" my router but when trying to connect, it just kept spinning around and never connected. I called Apple for help and they spent a long time having me try various things and finally gave up and are sending me a new iPad, assuming that the iPad was defective. (It hasn't arrived yet.) I know I had the correct password--it works for my other devices. Is there any router setting that I should check out? Is there a limit to devices that can use the same router? I'll be really embarrassed if the new one doesn't work!