A brand is weak when it cannot communicate its values – or when it is incapable of asserting a price premium for the added value it offers.
Why does this happen? Weak brands usually have a low profile, are insecure, and unclear in their communication. At their brand touchpoints they do not deliver a clear brand promise. The result: Neither employees nor customers know what the brand stands for, so they don't find it attractive.
Brand weakness is often a mark of those brands that consumers consider to be interchangeable. For many people, for example, butter brand x and butter brand y are interchangeable; the same goes for cotton pads or flour brands. Airlines are interchangeable when travelers look for a cheap direct flight from A to B. The focus here is not on brands like easyjet or Ryanair, but on the transport. So if a customer has no preference for an airline, its brand offers no discernible added value and can be called a weak brand.