Multiple faces must be contiguous and the contiguous edges must be linear.
You are informed when selecting a face will result in a mismatch error, which can occur when:
An edge between selected faces is nonlinear, and
The UV axes do not align along the nonlinear edge.
You can select another face or another type of mapping, such as projection mapping.
The order in which faces are selected can impact whether a decal maps correctly across multiple faces.
Non-contiguous faces do not map correctly.