I am working in a PSU company as an assistant engineer in maintenance department.I want an NOC from

I am working in a PSU company as an assistant engineer in maintenance department.I want an NOC from HR department to apply for another external employment. HR department says that NOC will be issued to me only when the format of NOC is duly filled and forwarded by my controlling officer to HR department.

But my controlling officer is denying to sign on NOC form without giving any proper justification. I know that according to company employment policy maual I am eligible in all respect to to get an NOC and HR department is also agree with me and renders my requirements legitimate.I don't know why my controlling officer is showing this kind of mischievousness. please suggest me what to do for getting NOC.