Scroll wheel rotates pieces. Holding shift prevents snapping - maybe you're accidentally holding shift? Getting pieces to line up to snap can be a pain at certain camera angles, often a temporary ladder can help. For finicky pieces where you're building back downwards sitting can help you target underneath to snap, but your character stands back up after every placement.

In short, if you're not targeting a piece properly it won't snap - scaffolding can help you target properly.