(2006) attribute this loss of per-formance to two factors: (i) in the genre of newspaper texts, which the Penn Treebank is based on, questions are not a high frequency syntactic construction, and (ii) if wh-type constructions occur at all in the Penn Treebank, they predominantly involve relative clause constructions or indirect questions, but not unembedded questions.
a parser trained on Penn Treebank data, routinely misanalyses unembedded questions as these other two construction types.