Where are you storing the ringtone file?
It helps to make a file tree on your card like this
\SDCard
\\Media
\\\Audio
\\\\Notifications <ringtones for notifications such as text/email go in here
\\\\Ringers <Ringers for phone go in here
Also, there's known issues of losing the custom ringtones every time you mount the card as a drive to the computer. If you put the tones in the folders like above, then it doesn't seem to affect it!