Some Devices Cannot Connect To Wireless While Others Can?

During the summer we had three laptops and an iPod. Two of the laptops and the iPod were having trouble connecting to the Internet, but I thought that was because we had too many devices but I looked it up and most sites said that wasn't true.

Now we have two laptops and the iPod and there are still problems so I thought it was a modem issue. It's a Webstar DPC2100R2 and when the Up and Down lights go off our wireless has trouble but if I use cord which goes through a D-Link DIR-615, the Internet is fine. When the lights go out I usually turn off both devices and turn them back on after about a minute and that usually works.But then, the lights didn't come back on so I called my Cox, my provider and they did something that fixed for about a week then the two lights went out again. Yesterday I reset them again and my Acer Aspire connected and so did my iPod but my HP Pavilion couldn't so I reset it again. Now my HP and iPod work but my Acer doesn't even though I've reset it multiple times.

What's weird is that the HP never has any trouble connecting, even when the lights aren't on or when my iPod, Acer and my sister's Dell couldn't.