There are a number of reasons why the red cover will not lock into place and these normally relate to the clamp being incorrectly assembles or positioned. Firstly check the order in which the arms have been arranged and that the spring washer has been orientated correctly.
If these are both correct, then ensure that when the hexagonal nut that secures the arms together is tightened, it is positioned in the centre of the wheel. Please refer to the attached fitting instructions.