Since I didn't get a response back quickly, I went ahead and typed you up a respons concerning what might constitute a difficult labor. It should answer your question. If it doesn't, please respond and I'll answer as soon as I can.
Let me do some explaining first. A puppy is in a sac of fluid. to which the placenta is attached. In some cases, the sac breaks as the puppy moves through the birth canal. In many cases, the sac is intact at birth and the mother bites the sac from around the pup and proceeds to clean the pup and get it breathing. The placenta usually follows the pup pretty closely but not always. You should have a placenta for each puppy though, so pay attention and be sure none are retained by the female.
If your female has been or continues to struggle to deliver a puppy for more than an hour, you will want to have her seen by your vet as the puppy is probably stuck.
If your dog is actively straining to deliver a puppy for more than 20-25 minutes, or there is longer than an hour or so between puppies being born and you feel there are more puppies, she needs to be seen by a vet.
Sometimes puppies are too large to be born and get stuck. They can also experience uterine intertia and you need a visit to determine if a shot to restart labor is needed.
It isn't unusual for there to be from 10 minutes to an hour between puppies being born. So keep an eye on the clock and record the time each puppy is born so you can accurately judge when a vet visit may be needed.
Sometimes a female will not push in the house due to house training. Taking her outside with a towel to wrap the puppy in if it should be born can be attempted if you feel this is the case.
It is possible for a mother to not deliver all her pups. Usually when a pup is retained, a dog does not act normal. They may appear restless, distressed, actively straining, lethargic, non-interested in their pups.
However, once your dog is eating, drinking, urinating, nursing her puppies, cleaning them and basically acting normal, she has most likely delivered all her puppies.
If you feel there is a puppy still inside your female, then you should take her to the Vet to be sure. It's a good idea to take her and the pups in anyway for a post whelping checkup to ensure all placentas were delivered as well
Here is a site on this information.
I hope you find this information helpful.