You probably do not select a right partition tool for your partition. In fact, I have ever tried to extend my system partition as it is nearly full. But, as I try to extend it in Disk Management after deleting a useless partition, the “extend” option just cannot be used. Therefore, I finally followed many recommended on the internet and tried many efficient partition management programs, such as:
IM-Magic Partition Resizer Free
EaseUS Partition Master
http://download.cnet.com/EaseUS-Partition-Master-Free-Edition/3000-2248\_4-10863346.html
GParted
http://gparted.sourceforge.net/
All of them are great tools and have eventually extended my system partition successfully. Till now, I still can clearly remember the extremely user-friendly interface of the first partition tool.
Just go select one!
Note: Do not forget to back up everything important in case of data loss.