hi Sharon, i've noticed that if my fob is near my iphone, the signal blocks and does not open the car! strange but true, so maybe keep the fob separate from your phone. i sometimes get this. i usually do the following:
get yourself some spare lithium batteries, and try to keep a note of when you last changed them. keep the batteries in the car
if the batteries are fine and you get that message. simply hold the fob to the start button and start the car as normal.