Cannot reduce virtual disk

I have one virtual disk configured at 100GB as an expanding disk. When I stop the virtual machine and try to adjust the size of the disk - reduce it - it looks like it is doing something but when I return to the previous screen it is still 100GB. I try reducing by different amounts, 5GB, 10GB, 30 GB - no luck. In Windows it says that I am using 47 GB of 73 GB. If I try to turn off the expanding disk option it gives me an error and says there is not enough disk space to do this. I have 64 GB free on my hard drive. Any thoughts. Thanks.