@OmSigDAVID,
As far as secret agents go, the
Abwehr was appallingly inept. The English rounded up all (or almost all--if any were left, they kept their heads down) German agents and "turned" them so that they were feeding information to Germany that the IGS (Imperial General Staff) wanted them to have.
It was easy to get Hitler to believe what he wanted to believe in the first place. One such example was Patton riding around England in a jeep, along with dozens of other jeeps, mimicing the radio traffic of an army in encampments. The Germans considered Patton to be the most dangerous American officer, and they wanted to believe that the Allies would be stupid enough to land in the Pas de Calais--Hitler in particular. So, while the Allies were chewing up the German 7th Army in Normandy, 15th Army stayed in place in the Pas de Calais, waiting for the invasion which would never come. In fact, 15th Army stayed in place until they were nearly cut off by Montgomery's army.
There are few things easier to do than to convince people, even intelligent people (which i pesonally don't believe includes Jodl and Hitler) that what they want to believe is true.