I would not expect any problems over a 15 metre range if the antennas are oriented properly. How are the two antennas angled relative to each other when transmission is lost?
Another thought is that there may be an obstacle (maybe even the Arduino itself) in the signal path at some parts of the rotation.
A loose connection is certainly a possible explanation. The only solution to that is to check EVERY connection very carefully. If the fault always happens at the same angle that might help to identify the problem.
You have not posted your programs. How many retries does your nRF24 make before it gives up? Do you know if it is the message or the acknowledgement that is failing to get through?
...R