Show us a good close up picture of your current setup.

When you have everything all tight, is the gap between the clamp all the way closed. Or if there is a gap in your seat tube, is that all the way closed by the time you are finished tightening your clamp. If either, then you need to figure out why that gap isn't there. Once the gap is closed, the clamp can't get make the seat tube any tighter on the seat post.

It's also possible that if the bolt you are tightening is a shoulder bolt, then other things that are wrong, might have the nut bottoming out on the shoulder before the clamp is fully tight. Similar can happen with other types of bolts and fastener designs.