Common causes
Joists that were always a little light for the span will sag over years. Moisture in a crawlspace can decay or weaken wood. A beam pocket can crush. A post can settle. Someone may have cut a joist for a duct or a tub. Overloading a room with a heavy tub, a safe, or storage can show up as bounce or slope.
Settlement of a perimeter foundation can also tilt a floor. That is a different repair than sistering joists. Treating the symptom in the floor while the footing is still moving is how people pay twice.
What Bob looks at
Access to the floor system, member sizes and spans, bearing, moisture, and whether the slope matches a foundation or a framing issue. You get a written reading of cause, not a product pitch for jacks or sistering you may not need — or that you do.