The usual diagnosis
Post bolts work loose in the rim joist through seasonal cycling; on decks built close to irrigated planting, the post base softens from held moisture. If tightening no longer holds, the clamped wood is the problem — worth catching while it's one section, not the deck edge.
Next step
Each loose post at its base, the run from the side, a short video if the post rocks — through the form for a range.
Railing mechanics: see the service page. City rules: see the Mountain View page.