In all the time I have attended meetings (the last 10 years or so) I have only ever experienced once instance where a sister asked the brother conducting the book study a question. Apparently she felt she had misunderstood one of the topics so she wanted to get it cleared up. Well the conductor looked at her like she had just committed the grossest sin possible! He looked at her and said "well sister _____, that wll be your homework assignment for this week, to look up your question using the WT library and report back to us your findings"
Wow I just about died of embarrassment for her. Poor lady. Never before or after that incident do I remember anyone else asking a question. My thoughts are it's probaby because the conductor doesn't want to be caught in a situation where he doesn't know the answer to the question. If you have questions, ask them on your own time, not on the WT's time!