A big issue is that they're reticent to be "that person" who monopolizes the time with their questions. They're also concerned about revealing their ignorance to their peers - despite the probability that their peers likely have similar ignorance. Hence this is why you're getting 25 students attending an optional review session, all hoping someone else asks the questions. An added wrinkle is that they're probably hoping you reveal what's on the test, and don't want to inadvertently waste time asking questions about things that are not going to be on the test.
Just because it's a review session for the explicit purpose of asking questions doesn't mean that your students aren't still students. As you're probably aware, students are (generally) horrible at in-class participation unless you explicitly construct the environment to encourage it. So I'd recommend pulling out the same bag of tricks you use during normal lectures to encourage class participation.
One I like is the awkward pause. We, as lecturers, tend to fill space. Typically it's less than a second between asking "Are there questions?" and then saying something else. You need to wait a good 5-6 seconds after asking before saying your next thing. This gives students time to organize their thoughts and get up the courage to respond. The length of the pause is going to feel awkward and you'll need to fight the temptation to fill the silence. -- That's a good thing. The students will feel the same awkwardness and feel the same temptation - and hopefully succumb by asking a question.
You also should lower the stakes. One reason they don't ask is because they're afraid of looking like the unprepared one to their peers. (Yes, despite the explicit purpose of this session is to prepare them.) You need to re-phrase the purpose of the session such that they know it's not just for the bumbling incompetent. This can be rather simple:
- Anyone have any questions? (*mumbling* "No.")
- So we don't need this review section? (*murmured disagreement*)
- Everyone has this stuff down cold? You are all going to get perfect scores on the test? (*vocal disagreement*)
- There's not any topic you want to know better? (It's important to emphasize the "any" here.)
At this point I'm guessing that someone will ask a question, or point to a particular topic they think they should know better. You've just lowered the stakes. It's no longer about playing catch-up for the woefully unprepared, it's now something that's useful for anyone who isn't confident they'll get a perfect score (which is probably everyone who showed).
Take an anonymizing approach. They're worried they'll make a fool of themselves in front of their peers or you by revealing that they're ignorant. (Or worse, revealing that they think they're ignorant on some trivial point.) Remove that fear by asking them anonymously. Have them write things down on pieces of paper, and pass them forward without names. Directly asking for questions might not help much if they don't have well formed questions, but you can get around this some by providing a prompt like "Write down what you think the three most important topics for the class are, and how confident you are on your knowledge of them on a scale of 1-5."
You can then get the students' responses and quickly skim through them for topics where a number of people are expressing doubts. (Even if it's only a 4/5.) Then present that topic to the group as one where a number of people are struggling. This will likely prompt people to ask questions about the topic. But if not, you can always take a "peer teaching" approach by asking someone to volunteer a summary of the topic, which should segue nicely into questions. Even if not, you can summarize the summary, highlighting things that they got right and filling in the blanks where things are missing or slightly incorrect. (To encourage participation, be sure to emphasize points where they got things right, and minimize drawing attention to where they got things wrong while still correcting the summary.)