I take it you didn't change seat post.

Try another QR or a simple seat bolt. If they work, then you're done.

If you have tried other bolts and QRs and still comes loose then you will need a new seapost. Either the post is not the right diamter, even though the post says or the frame is a bit bigger in diameter than it should be.

I doubt this is the case, but either way you should know.