So it seems you know the hardware is sound, and it is capable of sending and receiving reliably.
As the next step, I suggest you make the radio settings in your sketches identical to the ones in the demo that worked.
I'm skeptical about your changes to the retry interval given this comment in the header:
/**
* Set the number and delay of retries upon failed submit
*
* @param delay How long to wait between each retry, in multiples of 250us,
* max is 15. 0 means 250us, 15 means 4000us.
* @param count How many retries before giving up, max 15
*/
void setRetries(uint8_t delay, uint8_t count);