A child with a very restricted short-term memory may have difficulty understanding long sentences.
I actually experienced a similar situatiion in Latin II class. I did well with Caesar who came, who saw and who conquered Gaul; and wrote about it in short sentences.
But when I enrolled in Latin III (buoyed by my apparent success in Latin II) I was in for a "Custer's Last Stand" experience. Here we read the works of Cicero who never wrote a short sentence, I am sure, under two pages long. I could never get a complete sentence into my all at once head.
By the time I got to the verb phrase through all the relative clauses etc., I had totally forgotten what the noun phrase had been, not to mention the information that was in all the subordinate and embedded clauses.
My short term memory, I believe, would have to have been 21 bits to get the whole Latin Cicarian sentence.