In the last couple of days, I've found that JPluckX (2.1.1) fails every time it tries to get the Guardian's AvantGo site (http://www.guardian.co.uk/avantgo/) but that Plucker still can.

It's not a huge deal, but I was ready to forget about Plucker and move 100% to JPluckX -- any ideas why this might be happening?

I also use Plucker haven't had any trouble with the link you posted though.

How exactly does JPluck (dis)behave?

JPluckX just logs its attempt to get the Guardian and then reports "failed". It's rather short on detail I'm afraid.