I think it is common for kids that age not to be able to answer a why question. I have a 3 1/2 year old in my dc who cannot really answer why questions either (though she sure likes to ask me "why"..LOL). She also does not respond when I say hi to her in the morning...but I think that's because her parents don't require her to...so I let it go. When she is with me during the day though I make sure she says hi back if someone speaks to her...sometimes parents don't realize they need ot require their kids to say "hi" "bye" etc or they will not learn appropriate social behaviour. Ruling out any actual problem with the child in your care (like autism), I would not worry too much as it may just be that her parents have not trained her to respond appropriately. I would just use gentle encouragement and lots of praise when she does answer (you can feed her the answer until she gets it..."Now you say...it's good...etc").