First bit first:
Similarly, most of the religious are not trained theologians. They're average theists who want to share their faith with others, and consequently most are only going to be able to answer questions to their own level of understanding or articulation. It may be that the theist understands something but lacks the terminology to explain it properly and thus has to use a roundabout approximation, or it may be that you ask a question that they've never considered in their day-to-day faith. It may be that they panic and decide to ass-pull or shift the topic to an area they know more about rather than put up a wall of "I don't knows."
Agreed. But it doesn't help their case at all - especially when so often they post from a position of definite authority, then go vague when pressed....then a page later are back with the definite authority. I'm sure you can appreciate how, when theists switch like that, they perpetuate the impression that they are deliberately avoiding answering the questions posed.
So it could be that the answer to "Why is it so hard for believers to answer a direct question?" is simply "they are unable to". Cool. Fine. No worries.
But to echo something Hatter said: if they can't articulate their faith, then they should expect the brickbats and yes, even insults, when they step into our house and say "I'm right, you're wrong, and I can't prove it or even articulate I, but I'M RIGHT!!!!!"
And, I would also say, if they wish to apply any religious concept into the wider world - gay marriage, to pick an example - then I would expect the proponents to be able to not only quote a line of scripture that supports their views, but also to be able to answer damn near all of the other questions we may have about the background of their faith.
Anyhooo.....on the read the second half!