I'm having a similar issue with my 8 month old. She can hold it for 7 or 8 hours in the crate without a problem, and she'll sleep when we're home at times for 2 or 3 hours. But when she's awake and active and out of her crate, she asks to go out almost every hour, sometimes half hour. At first we thought she was just bored and wanted to go out and find sticks, so we would push it longer, but a couple times she was asking really insistently to go out and ended up peeing on the floor. When we're outside at the park she'll pee when we first get outside, and then often not pee again until an hour or more later when we get back, and then only because we bring her to her pee spot and tell her to go.
She's been to the vet recently and I don't think it's a health issue, but it sure is frustrating that she can't hold it sometimes.