When running Partition Assistant, we suggest you can close all other applications including the antivirus software and the firewall. The advantage is that your desired operations can be finished all right with no required restarting computer.
Please first check or defrag your disk as bellow:
Please right click the drive that you want to resize in "My Computer", and click Properties-> Tools-> Defragmentation and then click "Defragment now…" You can defrag the partition for its performance.
Please right click the drive that you want to resize and then click Properties -> Tools -> Error checking -> Check Now… Choose both options, and click Start to check. If have any problem is found, you can enter "chkdsk.exe /F C:" into CMD to fix these errors.
The shrink partition needs some free space, so we suggest you to remain 100MB free space. If the free space is insufficient, you could move some files to another space first.