Finding cause of sagging floors

[RussellB]

I live in a timber house that sits on brick piers. One section of the house seems to be sagging as doors no longer shut and floor tiles are cracking. I purchased a long builders spirit level but I can't see anything obvious. There are bathroom and laundry tiles and carpet that may interfere with the reading. What's a better way to find what's causing this issue? It covers an area of 2-3 rooms.

[rwbuild]

Age of house?
Soil type?
Termite activity?
Any permanently damp/wet ground around house?
Distance from ground level under house to floor level?
Is there a continuous brick perimeter wall around the house from ground to under floor framing?
Any substantial ground works adjacent to your property?
Time span from first noticing a problem until doors won't shut?
If you can answer these questions then a plan of attack can be established.

[RussellB]

Weirdly my reply to this disappeared. 30 yo simple country weatherboard on gentle sloping farmland, piers 0.5 to 1.2m, no brick walls, no water source/ponding nearby. Been an issue over 10 years but getting worse.

[rwbuild]

PM sent, await your call