Well it sort of depends on why they're not playing. Sometimes it's just a matter of finding what they like. Not all dogs like tug or fetch but might enjoy playing chase, roughhousing, or chasing a lure on a flirt pole. Or you're not playing the way they like. A lot of people can't get my dog to play tug with them for an extended period because they continually yank the tug out of his mouth and he doesn't like "losing" over and over. With me we can have a nice long game because I pull gently. Some dogs need their person to be very animated and physical while some dogs hate that. So it can just be a matter of knowing what your dog likes.
Then there are some dogs that don't know how to play, especially if they were neglected when young so never had any toys or anyone who played with them while they were at their most playful age. My friend has two pet store dogs that were purchased as older puppies, close to a year old, and neither of them know how to play with toys. I have never personally experienced this but do know people have luck teaching them how to play again.
Some dogs don't play if they don't feel well, a lot of people who have very overweight pets that go on diets comment on how much more playful their dogs become once the weight is off. So any sort of discomfort or conditions that cause fatigue could make a dog not want to play so much.
Then there are just some dogs that are not that playful, it's their temperament. Personally I wouldn't force the issue. Find something the dog does enjoy and do more of that.