If the quality of the rendered file is lower than what you can hear in FL Studio, there's a bunch of different things that may cause this:
-Make sure that the render settings match the desired quality, pay attention to the bitrate, sinc rate etc. I do not recommend using 512-point sinc, though! Usually you cannot hear any difference between 64-point interpolation and 512-point. Use 512-point only if you are 100% sure that the track is mastered right.
-Check if you made any mistakes in the mixing/mastering process. Lower the master channel to -6db and slowly tweak every separate channel so that there are no loud peaks. Pay attention to stereo separation, it can drastically improve the soundscape.
-Go into the settings of every separate plugin you are using and (if possible) set the quality to full. Be warned though, this will consume a lot of memory and you may run into playback problems within FL Studio. Adjust the buffer rate accordingly, and do this only when you are sure you want to render the track.