To be fair to the OP, she had a problem getting through - and she was only on transit. Most of us get through ok (you have, I have too), but MMV (as in YMMV). Moreover, I never take for granted that passport control for any foreign country will let me through no matter how many times I might have gone through in the past. So it's understandable if she's gunshy about actually entering China to walk on the Great Wall on her return trip to wherever she started from. Hopefully with a little more info from the OP we can diagnose what went wrong. Or maybe it was just her unlucky day (like the person who paid for and got approved for TSA Precheck, but that is the one day the BP shows the dreaded SSSS).