Let's see...
If you're using a DS game to connect to Wifi, you might have to go into the DS game itself and set up Nintendo WFC that way (instead of accessing it via the 3DS System Settings). This might not be the issue at all and you might not even have to.
With the settings you set up already, does it pass the basic connection test (that it does whenever you adjust and save a connection's settings)? If it doesn't, you may need to tweak that itself, such as the wrong network key or wrong type of security (e.g. WEP instead of WPA). On that note, I think DP (and maybe Pt as well) doesn't work with WPA security.
Another bit is, on the 3DS, to do anything wireless/Wifi, the wireless switch (on the side) needs to be turned on (if it is, there'll be a little orange light next to it). If it's not on, any sort of communication will auto-fail.
That's all I can think of off the top of my head right now...