I'm drawing with bezier set to straight lines and occasionally some of the lines won't move. There is a handle on one side of the node, but not on the side that won't move. What to do?

You can drag a handle out of a node, if it doesn't have one. Put the mouse over the node (so it turns red), hold Shift key, and drag out the handle.

If you have a curved segment that you need to be straight, you can retract the handle, to make it straight. Hold Ctrl key while you click once on the tiny circle at the end of the handle (also turns red when the mouse is in the right place).

In that case my usual solution is rectangle-selecting both nodes of the segment, set them cusp, set segment linear, then set it back to curved.
It positions handles in 1/3 distances, which can produce a "natural looking" bending.