I have the exact same problem with actuators. I tried my best to resolve the issue just like parachutesj but finally gave up mysensors for actuators and replaced them with wifi connected esp8266s. MySensors is still great for sensors, especially battery powered ones.
Finally managed to fix it. It was an issue with the battery booster. In the PCB battery booster is connected to the MCU only and the radio was powered directly. Fixed by removing the booster. Now both mcu and radio 3.3v pins are connected together. Battery booster can be connected externally.
@martinhjelmare Thank you for taking the time to write a very informative reply. It resolved the problem. At this stage my aim is to use it as a simple on / off type switch to control the screen from automation scripts. I used delay instead of wait since wait() didn't power up the radio automatically to receive messages. But with the modified script I use wait() since it sends a status message at the end and it wakes up the radio again.
I used relays. Most commands sent to actuators resulted in NACKs. But sensors were fine. I have tested different Arduinos, nrfs, circuits with an RPi gateway as well as a serial Arduino gateway with no luck. Problem temporary goes away when the gateway is restarted. This is true for both RPi and serial gateways.