youre doing something wrong. might just be volume, theres an illusion created when somethings loud it sounds fuller and thicker, so when you suddenly hear it quieter it doesnt sound the same (but it is)..might be that.. i dont use fl anymore so i cant help too much, might be a vst thing
well depends on what version FL your using if its 9 it could be in the limiter. You can turn the gain up a little in the limiter and raise the ceiling to give your sounds a little moar punch. If you go to your FX its in the master channel as the last FX tab i think green is your gain and the grey is the ceiling or whatever its called. If that dosnt fix your problem than its all in your mix down/eq etc.
maybe you should check the mixer's master channel. limiters on? trake it off and mix/EQ it on your own for the best!
If you got questions hit me up... I'm producing tutorials in FL Studio as well so feel free to ask me per PM