Consider a person reading a mathematical proof, then each syllogism from it's antecedent maybe understood by that person, yet they may find it difficult to understand the whole proof. At times however, a person maybe able to understand the proof as a whole, and in that case, provided they know the proper fundamental deductions, can deduce individual steps of the proof from it.
I hope the reader relates to this example I gave. In which case, I ask, why exactly is it difficult to move from understanding of the fragements to understanding of the whole? What is the missing piece?