Bryant Central Air Unit does not turn on without a little "tap"
 

When I set my thermostat to turn on my Central Air unit it will not
turn on after it has been off for a while. I have to go up into the
attic and remove the access panel cover and push it back on a bit
forcefully and it will turn on immediately and runs as expected. I
thought perhaps there was a sensor that keeps the unit off if the
panel cover if off, but I could not find any such sensor.

The next time this happened, I decided not to remove the panel cover,
and just tapped the unit gently (think the Fonz and the jukebox in
Happy Days). It immediately turned on. I cehcked the connections of
the wires on the spade connectors and none seemed loose to me.

Also, the filter has been changed very recently and is not showing
heavy build up of dust, so I think that can be ruled out.

Any thoughts on this one I am stumped.

Thank you for your time,

Buk