2. They get left behind trying to work out what a previous word meant
3. They just don't know the most important words
Again, doing vocabulary pre-teaching before each listening as a short term solution and working on the skill of guessing vocab from context can help, but please make sure that you practice this with words that can actually be guessed from context (a weakness of many textbooks) and that you work on that with reading texts for a while to build up to the much more difficult skill of guessing vocab and listening at the same time. The other solution is simply to build up their vocabulary and teach them how they can do the same in their own time with vocabulary lists, graded readers, monolingual dictionary use etc.