A child may grasp something quickly but that does not necessarily mean they understand it deeply. Being stuck helps pupils to learn the things that can't be taught: inventive thinking and creative problem solving. That requires non-standard, non routine problems - problems that can't easily be solved. To secure greater depth we need to give pupils the right: challenges, questions, opportunites and teaching styles.
It is also important that we create a culture where it is ok to be stuck, ok to try and fail, ok to think inventively and safe to take risks.