In such cases, there are a large number of possible labels for each token, so picking the right one simply by chance is unlikely; the parameter space tends to be large; and devising good initial parameters is difficult.
it is unsurprising that the unsupervised (or even weaklysupervised) learning of a Combinatory Categorial Grammar (CCG) supertagger, which labels each word with one of a large (possibly unbounded) number of structured categories called supertags, is a considerable challenge.