In FL a filter can only be applied to a sequence of samples/loops, with each sample/loop given a specific value. In this way you can change the envelope. That's one of the shortcomings of FL: effects like panning, eq. or filters can only be applied to the ‘buttons’ in the pattern (and if I’m wrong…let me know !!!). If I want to adjust the envelope of a loop or long sample I use an editor. Sound Forge works excellent for me.
Maybe an idea to create a FL forum?