A patio that has gone wavy, sprouted a low spot that puddles after rain, or grown a lip you catch your toe on was almost certainly built on too little base. Not bad stone, not bad workmanship on the surface — just not enough underneath.
Here is the full stack, bottom to top. First, excavation down to subgrade: the clay comes out, and it comes out deeper than most people expect, six to eight inches below where the finished surface will sit for a walkway and more for anything a vehicle touches. Then geotextile fabric over that subgrade on soils that need it, so the clay below and the aggregate above stay separate instead of slowly mixing into mud.
Then the base itself: crushed aggregate placed in lifts of a few inches at a time and compacted between each one. That phrase, in lifts, is the whole trick. Dump eight inches at once and run a plate over the top and you have compacted the top two inches and left the rest loose, which is exactly the same as not compacting it at all — you just cannot tell until the first wet spring.
Above the base goes a thin screeded bedding layer, an inch or so, struck flat with a screed board rather than raked by eye. Then the pavers or the stone, set tight, cut at the borders rather than filled with slivers, and locked at every free edge with edge restraint so the field cannot creep outward under foot traffic.
Last comes joint sand, swept in and compacted through the field, with a second pass after the first settles. Skipping that second pass is why joints look half empty a month later.
None of this is exotic, and none of it is fast. It is roughly two-thirds of the labor on a patio and all of the reason yours is either still flat or not. When you read an estimate, look for the excavation depth, the base depth and the words compacted in lifts. If they are missing, ask. A mason who does it properly is always happy to tell you exactly how deep they are digging.