Is the extension wired pin-pin (no crossovers)? Other than that, I wouldn’t expect 1.5 m of extra cable to affect it, even though we know very little about the inner workings of the device.
It’s established fact that the state of the green led on the pulse sensor doesn’t necessarily relate to the state of the output going to your emonTx.
Where is the meter & pulse sensor - are they in a relatively dark place?
Take a look at these threads, they might give you some clues.
From what you have written, I wonder whether the input to the emonTx is remaining HIGH due to ambient light that is enough for that but not enough to light the green LED.