Maybe go back further and find out why they failed in the first place. If they failed because they weren't interested, first you have to generate enough interest for them to pass. Or perhaps you should talk to each one privately and find out if there was content that was too hard. Failure of any sort makes for shame and lack of confidence, then follows motivation. If you can address the bit they failed with then their enthusiasm might return and then the interest will already be there.
In short, people who are interested generally ask questions anyway.