I suspect the previous answer will rarely apply to the problem as reported by OP. Most likely the floor isn't "solid" hardwood (ie - it's probably not 2-3cm thick floorboards laid directly over joists that have sagged).
I've often seen what OP describes on "click-lock" flooring, where interlocking 6-10mm thick panels are used to "tile" the area. The panels are usually laminate rather than "real wood", but according to that Wikipedia link it's often called floating wood tile in the US, and some artificial composites are so good you might think they're real wood anyway. Whatever - the problem can apply (though less commonly) even with real "wood strip flooring", as it's often called in the UK.
The American name itself suggests where the problem lies. These "secondary" floor coverings are supposed to float, meaning they should not actually be fixed to the flat surface underneath. In the UK, most modern building have 8' x 4' chipboard sheets screwed into the joists, and the "floating" floor lays directly on these sheets.
Actual floorboards (generally older buildings in the UK) are usually lengths of softwood about 6" wide spanning the entire room. If a floating floor is laid on these, they're normally covered first with a layer of thin (4-6mm) plywood sheets, to get the required flat surface for the floating floor.
With these types of flooring, it's essential the entire top covering is actually able to float - whether it's real wood or a composite material, temperature and humidity changes over time are likely to cause expansion. The hollow/spongy patches aren't caused by sagging. The floor is trying to expand, but is being pinched at the edges because there's not an adequate expansion gap where the floor meets the walls, so it buckles upwards.
Because "click-lock" flooring looks easy, people often do it themselves - or employ an incompetent handyman who does a job that looks good until he's been paid, but causes problems later.
In every case I've seen, the solution involves taking off the skirting boards (apparently called base or mop boards in US/Canada). The floating floor is supposed to go under the bottom of the skirting, but still have about a 1cm gap between the flooring panels and the actual wall. If there's no gap, chisel one out. If the flooring panels were actually butting hard against the front edge of the skirting, saw/chisel the bottom of the skirting so it can go under. Then just re-fix the skirting.
Sometimes the "pinch points" may not actually be at the skirting/wall. You may have a fitted kitchen, for example, where the base units are solidly fixed to the underlying "subfloor". But the same basic principle applies. There will probably be quadrant (scotia) strips covering the vertical/horizontal join. Prise/chisel these off to check whether the floating floor is being trapped against something under there - if so, just adapt the remedy outlined above.