In response to your open question, "What are some reasons why students may have trouble exploring a text in depth?"
A few potential issues come to mind in regard to this question. First, as you questioned, I do believe it is directly related to vocabulary and level of comprehension. In working with high school students, it appears the level of vocabulary has diminished. However, I've always been a reader and my parents always made me look words up in the dictionary that I didn't know. It never occurred to me that they may not know, but it didn't matter - I looked them up, and I learned. I learned how to read phonetically, and had a solid grasp of root words which enabled me to often figure out what a word meant in addition to prefixes, and suffixes, and through the context. I know through raising three girls that the methods of learning to read have varied over the years, but I believe the way I learned allowed me to make connections to comprehend texts even when I had not encountered a word before. Even now, though I can likely guess the meaning of a word with a fair amount of accuracy, I still look words up to see if I'm right (which I usually am). If learners don't know what words mean - the comprehension will suffer, and they won't be able to extract meaning from a text.
Additionally, the subject matter of the text is also significant. How engaged a student is in relation to a text can also make a huge difference. However, let's be real - we can't make everyone happy.
So what do we do when learners are on their way to college without the ability to comprehend texts, and are therefore less able to explore a text in depth? Knowing that this is happening, we would also have to admit we need to go back further to make changes to prevent this situation in future students.