If you've already made sure your connections are tight from the cable outlet to the digital box, rebooted and noticed no change, as @Gdkitty said, you'll likely want to have a technician come visit to investigate.
Not knowing which box you have, I can't tell you how to access the diagnostic screens. What yo need to do is check signal strength on the channels that give you trouble. The NextBox 3.0 is sensitive to poor signal strength. I would suspect a bad or incorrectly installed splitter, or just a loose connection.