Checklist
1) Make sure both units are communicating on the same global channel (not necessarily ch. 10).
2) If you are using both MIDI cables, you might try disconnecting one of them (the one that SENDS from your BOSS machine, since that one is not totally necessary for what you are doing at this moment). Though this is not a true solution, for now, it may be sending signals that may be causing an echo... so rather than address that problem now, simply pull that cable off, for now.
3) Without knowing the BOSS equipment, you may be in SONG mode which you do not want. Meaning, is there a way to set up the right drum kit you wish to use in Patch mode, without resorting to the SONG or SEQUENCER mode. If it is in Patch (program) mode, then you should be able to hear that program when your FL Studio sends the MIDI notes. FL Studio is your main sequencer at this point, and you do NOT want the BOSS to be acting like a sequencer.
4) There may also be an echo happening between the 2 devices. Roland is known for having problems with Echo with other companies equipment. Many manufacturers take the time to write about it in their own manuals. An echo is like sending the data twice (bouncing) and well, the bottom line is it causes communication problems. So read your manuals that address the term "echo" and make sure this is not happening.
Don't know what else to add at this point.