Usually if a dog won't eat I'll just hold off until the next meal. A lot of our dogs will be really picky eaters, especially at the start of the stay. We can't leave food down (our dogs would eat it), so at meal times, we'll put them in a room on their own with the food for 10-15 mins, then pick up whatever they don't eat. Some of our dogs won't eat unless you stay in there with them and watch them. Some will only be encouraged to eat from our hands. But unless it's a longer stay and the behavior goes on more than a couple days, I don't worry too much about it. None of them are otherwise acting distressed (in fact, they're more than happy to eat any treats we have, and usually they really want to eat our dogs' food), so I'll let them eat when they choose. I've found especially dogs who are used to free feeding at home and aren't very food motivated take a while to adjust to a meal schedule, but they do eventually get it.
Tricks that I've used to get a dog to eat when they really need to: