First off you have to make sure the file is even making it to the memory card. Can you "see" the file on the card from your computer?
If not maybe their is an issue with your card reader. Unintall/reinstall if you suspect the reader.
Also, you can try reformatting the card on the phone. This will ERASE all data on the card.
The file will have to be in the My Ringtone folder to be seen by the phone.
Additionally, Samsung has (or did have) a 300kb limit on the size of the ringtones on their phones. Make it smaller and see if it works.
The only other thing I can think of that might make it invisible is if the file has DRM (digital rights management) in it.