technically speaking, DSi download play is the same as DS download play, and all iterations of DSes (even the 3DS) all talk the same wifi language, so there really shouldn't be any problem. seeing as you're having problems though, here's what i think:
1. you're using a flashcart, and the flashcart you're using doesn't do download play correctly.
2. the download play feature of that particular title is bugged (see Lord of the Rings Conquest for the DS. its local play is broken literally, even on retail. i have yet to hear of anybody being able to play this title on local wifi)
3. hardware problems. but if wifi hardware isn't working right, you shouldn't bee able to see any available download play titles.
4. interference. there may be another wifi device that's interfering with yours, so you see the download play titles being broadcast but couldn't download it.