I know I’m late to the conversation, but – @xiggi and @marvin100 – IME with students, it’s both/and, not either/or. (1) Marvin has a point that lack of comprehension often points to vocab weakness (and what I observe more is that they’re not aware they don’t know some of the words, sometimes; they fool themselves that they do, or they’re convinced that “smart people (they flatter themselves) figure out [word meaning] from the context” – except for the problem that they don’t know roots (having not taken Latin or Greek), and more shockingly they have often forgotten, or never knew, English suffixes differentiating nouns from verbs from adjectives. (Don’t know that -ity is a noun ending; unbelievable.) And therefore they cannot figure it out from the context.
(2) But xiggi is correct IME that the act of reading is done lazily and ineptly, so even when/if they have memorized vocab lists, they are neither understanding the structure of the sentence (a grammar & a reading problem), nor are they understanding meaning through sequence. (The next sentence builds on or explains the previous sentence.)
Much of the reading failures are based on modern faults and deficiencies: (1) little to no experience with sustained reading; (2) impatience with complete texts and the sequential reading process – due partly to Internet reading habits; (3) false expectations attached to skimming and other kinds of partial reading.
One of my students is like a human memory machine, but it was not until I showed him that he was mistaking the part for the whole (when reading) that his practice test scores dramatically increased.