If a puppy is slow to take that initial breath after delivery, the puppy will die unless you can intervene. You need to learn how to resuscitate a puppy.
First: Be vigorous. You cannot kill a dead puppy. You need to clear the airways vigorously using a bulb syringe. Clear the THROAT first several times, until the syringe removes no fluid. Then you clear the nostrils with the syringe. Do not clear the nostrils before the throat, this will stimulate the puppy to breathe in all the fluid. If you hold the puppy upside down when you do this it will help to drain the fluids away from the throat and lungs. If need be, you may have to swing the puppy. Lie the puppy on the palms on both hands, your forefingers under the neck/chest area and your thumbs on the back of the neck/ head area to support the neck. Stand with your legs shoulder width apart, hold the puppy up in the air and swing it down fast (SUPPORTING THE HEAD AND NECK) between your legs. This clears the lungs but also stimulates breathing. Try this 3 times. Be aware. Wet puppies are very slippery.
Second: Check for a heartbeat between the front legs using either your fingers or a stethoscope. If no heartbeat using your fingers perform chest compressions. A hard thump or flick to the chest may just be enough to start the heart.
Third: Rub hard with a dry towel. It is very hard to break a puppy's bones, they are rubbery. You have to rub hard to stimulate breathing, your aim is to shock the body into taking its first breath. Keep the puppy warm. Get a hot water bottle and place it on your knee and put the puppy on the bottle. It should be hot enough to be very uncomfortable on your legs. Rub the puppy constantly. Try using your DopramV. If you haven't got any DopramV, in a dire emergency, use a drop of alcohol (Brandy is best), but only spirits will work. The alcohol sometimes stimulates breathing. However this is the importance of the DopramV, this is what it was designed to do. Have someone hold a hairdryer over the puppy whilst you work on rubbing it. Be rough with the puppy: a stroke will not make the puppy cry. Your aim is to make the puppy shout out!
Fourth: The Breath of Life: Cover the nose and mouth with yours and breathe gently (remember lungs are only small) into the puppy.
When to stop: You have to go by your gut instinct. There is no law or rule as to when you should call it a day. Puppies have been brought back after 40 mins of what appears to be nothing/ no hope. Go by your gut instincts. Weigh up the pros and the cons of keeping going.
Drop Test: Do not declare a puppy dead until it is warm and dead. The body goes into a suspended mode when it is cold. Try to get the puppy warm before you decide enough is enough. To perform the drop test, hold the puppy belly down about 6 inches from a heated surface (padded) and drop the puppy onto the surface. If the puppy's legs spread out then are quickly pulled in, he is still alive!
Also a puppy will have no reflex if it is dead. Even a newborn (who have sealed eyes) will have a blink reflex if you touch their eye lids. A dead puppy doesn't have this reflex. Try the suck reflex, put your finger in the mouth of the puppy and see if it makes any attempt to move its tongue. Don't give up if these signs are still here!