Hello!
@fisher
Please have a look here: http://forum.mysensors.org/topic/2989/nrf24l01-range-of-only-few-meters/7
It solves my problems.

@epierre
The NRF24L01+ is working on an other frequency, it is 2400 MHz and not 868 MHz. I think to post http://forum.mysensors.org/topic/775/8-lamp-outlet-smart-plug-module/67 might be correct.

@epierre
petewill propose full wave antenna, please look here: http://forum.mysensors.org/topic/775/8-lamp-outlet-smart-plug-module/67

I also have such problems, the following video helps me. I use 3.3V pro mini.
https://www.youtube.com/watch?v=NpMnauHeR7Y