I suggest add a
sleep 10 or
sleep 20 in your
crontab and then try rebooting the Pi
@reboot sleep 20; /usr/bin/python /home/pi/RFThingSpeak.py &
This took me more than 6 months to figure out. Most of the
/dev/ may not be ready when you reboot. but adding a sleep should most definitely do your work.
I asked a similar problem on This Thread on Linux & UnixThis Thread on Linux & Unix and have seen many similar boot related problems on StackOverflow which also suggest using
wait before triggering your script viz. for SQL, Python related boot scripts.