This is probably because your DSi is upgraded to the latest firmware that your card doesn't support yet. Once your DSi console has been updated to v1.4.3 firmware it cannot then be downgraded. If you are using a genuine R4i 3DS card
(check on the r4dsrevolutioncards.com website by the way, they do a
pretty good guide on the different types of genuine / fake cards. Also
see r4dsrevolutioncards.com, they're pretty useful for R4i card
info) then you should be able to update the firmware of the R4i card
through the "Settings" menu of the R4i. You need to do this on a
1.4.2-or-below DSi console, or a DS Lite though.
NintendoDSCards.com has an info page and a video to show you how to
update a legit R4i card (NOT fakes) - click here to visit the 1.4.3 R4i Update
If you're using a fake card (such as the R4i Gold, R4i Ultra etc.) then
it's quite likely the card simply isn't supported and you'd need to get a
new one. Your best bet is to buy an authentic card and simply buy
something decent that's upgradable. DS cards unfortunately don't last
forever, and teams eventually stop supporting the cards - but the fake
ones are terrible at this to be honest!
I personally shop with dscardworld (www.dscardworld.com) and they're
great, but that's because I live in the US. I used to live in Sweden and
if you are in Europe then nds-gear (nds-gear.com) are just better if
you live outside of the US/Canada in my opinion, so pick depending on
where you live. Both of those shops are authentic retailers of R4i,
Acekard and Supercard by the way (probably others too but those are the
ones I've used).
If you're getting a card for a 3DS / DSi console I'd recommend a Supercard DSTWO
since it's pretty much just awesome and you can play Gameboy Advance
games as well on it. But if you just need a basic card or you're using
it on a DS Lite / DS, I'd get an Acekard 2i
. Up to you really, I think both of them also stock the genuine R4i's but they seem to be going out of fashion these days.
Hope I've helped, good luck!