Momentum gives you one new photo and quote each day at 4:00 AM local time. If your background is only changing once a day then things are working as they should.
If you notice that you are getting the same Background(s) throughout the week this could be the result of a couple of different things, please consider the following:
If you have an ad-blocker installed it could be preventing Momentum from downloading and updating your dashboard with new backgrounds.
If you temporarily disable your ad-blocker on your New Tab page and then refresh your page a couple of times your backgrounds should be updated.
A fall-back image will appear when the original image cannot be displayed for some reason or another. If you refresh your New Tab page a few times you should see your unique daily photo.