Levels climb again when the fan fails, a sump cover or seal is disturbed, or the house changes through an addition, a basement finish, or waterproofing. A working, maintained system holds its result, which is why Ohio says to retest every two years.
More detail
Watch the gauge. Ohio requires every active system to have a monitor, and a manometer has to be marked with its reading at start-up, so any change is easy to spot; liquid sitting level on both sides of a U-tube means the fan is not pulling. Fans wear out, and the EPA notes many last five years or more. Seals fail more quietly, most often after someone lifts the sump cover to service the pump and does not reseal it with silicone the way Ohio's standard expects. Renovations are the third cause: a new bathroom rough-in cut through the slab or a crawl-space addition can open a path the original design never covered. Retest after any of these. See is my system working.