I've been working on adding packet acknowledgment to Flutter's protocol. If you'd like to try it out, please see the instructions below. Note that this is not yet added to the Flutter API, so you won't get much from trying this unless you want to see how it works!
First install ArduinoLog to your Arduino libraries folder (you can get it in the built in library manager under "ArduinoLog by Thijs Elenbaas"), then download the experimental code here. Use the AckDebug example built in to that branch, flashing one unit with the #define TRANSMITTER line commented out, and one with it enabled. Then connect both devices by USB to a computer and open the serial ports that show up. You should see debug data in both terminals. If the receiver device doesn't want to sync to the master, unplug it and try reconnecting - I think there is a bug in the sync code that prevents device sync if a unit is powered on at the wrong time.
Let us know how it works for you!