Do you see something like this?
Mine (W10) is saying I could have the full 50% free-space if I wanted to.
Maybe you have some unmovable files blocking the upper sectors and need a defrag first
In any event, Disk Management has a mind of its own sometimes and is not very good even when it does work
Have a go with MiniTool Partition Wizard Free
If you explain a bit more what you are trying to do, we can offer extra advice
Note that messing with partitions carries some risk. Make sure you are fully backed up.