There is a dead spot on the tip of our antennas but it really does not matter all that much. At the distance you were out you could have simply had a low battery in either the plane or transmitter which somewhat decreases range, again not all the much, something on the plane that was vibrating that would easily decrease range and is more likely (called RF or interference so the obvious question is when did you last perform a range test with the engine running and what were the results)

But at 1200 feet out, I am surprised you had a decent amount of orientation. Even my big planes (80 inch or more wing spans) at 1200 feet straight out get kind hard to see and if the clouds are low or just late in the evening then it is even worse.

I am sorry about what happened but you may never know what the real cause was