When a brand is inconsistent and put out mixed messages then the consumer perceives it to be inauthentic and therefore doesn't believe what the brand says about itself. e.g. A brand trying to look like a luxury brand, but has dirt cheap prices makes the consumer question if what they're getting is actually premium. Other examples: "You say you're innovative but your products are very stock standard", "You say you're traditional and trustworthy, but your messages are very edgy and comedic."
To have a consistent brand requires a good understanding of your brand.