07 chevy impala heater intermittently inop

We have an 07 impala and my wife started the car and
turned the heater on,  but it didn't come on. She drove it to
the store and when she came back out and started the car
and tried the heater again it came on. It did the same thing
today.  She tried other settings for the blower motor and it
didn't work and flipped it to the A/C and still nothing.  It's
like it's not getting any power,  but then she stopped for gas
and when she started it up the hvac was working as it
should.  Any suggestions?