Skip the WD40, it's a mediocre lubricant and not a penetrant at all.
First -- when you turn the stem bolt, does the head come up? If the bolt head is still above the top of the stem, then the wedge down below hasn't released. In that case, DON'T try to rotate it. Tap on the top of the bolt. Tap harder if you have to. The bolt head should eventually drop, indicating the wedge has released. At that point, the stem SHOULD move readily.