Momentum gives you one new photo, mantra 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 photo(s) throughout the week, you may be seeing a fall-back image or your ad-blocker could be preventing Momentum from updating your page with new content.
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.
If you have an ad-blocker installed it could be preventing Momentum from downloading and updating your Momentum page with new background photos.
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.