I bought a used bike for my wife and got everything working except for one item. No matter how much I tighten the seat tube/post securing bolts, the seat post won't stay up when weight is put on it. This also means that if I put it at the lowest setting and tighten the securing bolt on the seat tube, the post still swivels (which makes riding more difficult).
The post might be the wrong size. Some dummies don't think that .2mm is much, but it's far too much difference to fit properly. You don't mention the type of clamp. Some traditional steel frames can be overtightened so much that the ears get stretched and will never work properly. Most modern frames have a separate clamp that fits over the top of the seat tube. Those can be damaged too, but at least they can be replaced.