Espresso machine has to warm up halfway through pulling a shot

I've had my espresso machine for a little less than a year. It's a DeLonghi EC155. It heats up fine (I assume so - a green light that means "ready" turns on, and I've followed the instructions and let it sit for 30 minutes heating before using it), but when I pull a shot, it makes the sound of the water being pushed through the grounds, but the green light turns off halfway through the process. I stop pulling the shot, wait for the light to come on, and finish pulling the shot.

Usually when I dispose of the grounds, they're very watery. Is there a way to make the machine hold heat longer, or is it a defect with the machine?