Did you remember to update the 'PLUGINNAME_supports()' function in mod/PLUGINNAME/lib.php to include:
case FEATURE_BACKUP_MOODLE2: return true;
?
Without that, Moodle won't even try to run the backup for your plugin. You could also double-check the directory permissions on the mod/PLUGINNAME/backup/moodle2 directory, to make sure your webserver can read the files.
Apart from that, it might be worth adding some 'echo' statements to the functions in your backup to see if they are being called (or, even better, install xdebug on your development server and use a IDE with an integrated debugger, such as PHPStorm, Netbeans, Eclipse, then you can put in a breakpoint and step through it).
If none of that helps, you could try posting some of your code here, as you've not given very much to go on so far.