Why can sealants sometimes fail in their application?

Sealants can fail in their application primarily due to their improper placement, which can lead to leakage beneath the sealant. This situation allows harmful bacteria to access the tooth surface, potentially resulting in decay under the sealant layer. For a sealant to be effective, it is essential that it is applied accurately to cover the entire groove or fissure in the tooth, ensuring a tight seal against the surface. If there are any gaps or if the sealant is not fully adapted to the tooth structure, bacteria can infiltrate, leading to issues that the sealant is intended to prevent.

While the other options may suggest potential problems with sealant application, they do not fundamentally address the critical concern of how proper placement is essential to sealants' effectiveness in preventing decay. Thus, ensuring correct placement is pivotal in achieving a successful sealant application that protects the tooth.