The possible reasons are:
1. The oil system leaks oil. The oil system and oil level shall be checked completely. Because sometimes the oil spill occurred in the container and is invisible, such as the pressure oil tube  inside the oil tank fractures, and the copper tube of the oil cooler fractures.
2. The oil system of non-return valve is  not closed tightly, part of the oil flows back to the oil tank  from the auxiliary oil pump.
3. The oil pump inlet filter is blocked or the resistance is too big.
4. Because valve opening of the oil system is not enough.
5. Due to the oil temperature is too low, which increases the viscosity of oil.
6. Main oil pump is in failure and the oiler is not working normally . 
7.Due to the  increasing of power grid frequency, the turbine speed drops, the high pressure oil and lubricating oil pressure will have a small decline.（js）