A floor that sags or bounces can result from normal flex in a long span, undersized framing, damaged supports, moisture, or a localized subfloor problem. The key is to separate an old, stable feel from a new change accompanied by cracking, slope, or deteriorated wood.
See whether movement is local or spans the room
A small soft spot near a bathroom or doorway may be damaged subfloor; a broad strip that bounces between walls can relate to joists and structural span. Mark the area so you can tell whether it worsens over time.
Look for moisture, cracks, and new slope
Staining, musty odor, soft wood visible from an accessible basement or crawl area, and new nearby wall cracks raise the priority. Do not cover a wet area with new flooring before the water source is corrected.
Do not cut structural members to investigate
Do not drill or cut joists, beams, or supports just to investigate. If access from below is available, limit inspection to safe visible areas; altering framing can turn a diagnostic question into real damage.
Rapid change or visible sag deserves professional evaluation
A floor that is worsening, visibly sloped, or accompanied by doors no longer latching and new cracks deserves structural evaluation. Move promptly as well when rot, termite damage, or shifted supports are visible.