For me, it is the sometimes archaic language used in the sūtra. Even though we do have wonderful resources—lexica, dictionaries, databases and so on—they will never answer all of your questions. And sometimes you come across a term which even very learned Tibetan teachers don’t understand, simply because they are so rare and peculiar to Sanskrit translations. They might not change the overall meaning of the passage—though in the worst cases they might—but to translate the text accurately we have to do our best to understand them.
To give you an example, we struggled with the term phyag dar ba for a while, which the dictionaries usually render with “sweeper” (phyag dar meaning “dust” or “rubbish”). The term occurs in connection with “advisors, ministers, soldiers and physicians.” To translate the term as “sweeper” did not make much sense in the context of the sūtra and all we had were our guesses—not very satisfying. Fortunately we had the Sanskrit original of the very passage in question quoted in Shantideva’s Śikṣāsamuccaya. This led us on the right track. It turned out to mean outcast or a very low representative of a particular kind. In a way, a person related to dust or rubbish in context of brahmanical thought. There was a connection, but we did not know how to make it.