Why does my Arduino DUE not recognize my external interrupt?
It is likely that the Arduino Due can't respond fast enough. 

An Arduino Due runs at 84MHz. Those interrupts are arriving faster than every 12 clock cycles. 

The interrupt response time for an ARM-CortexM3/4 is an absolute minimum of 12 clock cycles, just to start the interrupt service routine, and that is without it doing anything. That is made worse by the Arduino library's attachInterrupt mechanism. Guess 20 cycles and we might still be being optimistic.

Test this hypothesis. 

Either:


use a second MCU to generate slower signals which can easily be
speeded up until you reach the breaking point, or
if you have access to a signal generator use that, or
use a (PWM) timer on the Due to generate the input signal.


Edit:
I think you will struggle to synch with an external NTSC 7MHz clock, and interrupts will push it beyond feasible.

You might generate the 7MHz clock using a Due timer, and try to synch that timer with the external clock each frame or scan line instead. It won;t be perfect, but might be close enough.