AGC (Automatic Gain Control) is a system where curcuits are controlled by a level derived from the circuit itself.
The more signal that comes into the circiut, the more AGC is applied to "Turn it down" to it's working level.
You should by now have worked out that it is most likely to be the "signal level" coming into your Box. This has reduced, resulting in your agc not producing or requiring so much control..
BM:xmas: