have not received increment in pension for 20 years

My father was working in iron ore mines of Steel Authority of India Limited in jharkhand and retired from service on superannuation in July, 1996 at the age of 58 years. The pension was fixed at about 670/- per month. From that time, he is getting only this much amount till date and there is no increment even after so many pay commissions. What is the provision of pension for employees of Raw material division of SAIL or he should get the increased pension amount with pay commission's report. SAIL representative said that at that time, that was the rule for pension.Please advice.