There could be a myriad of reasons why it is not working:

1/ you're reading the wrong sector - try reading the FAT table, or have your code dump a few sectors.
2/ your code may have defects

If the MMC card is giving no error, one would expect that it is giving you the data you requested, as to what you do with this?


Unfortunately, you really can't expect us to diagnose your problem as we have little information.
Have a look at this web site, I've used FatFS successfully - it might be able to give you some information.

http://elm-chan.org/fsw/ff/00index_e.html