The operating pressure is too low.
(1) The amount of oil in the gearbox oil pan is insufficient;
(2) Oil leakage in the main oil pipe;
(3) The gearbox filter is blocked;
(4) The steering pump (or hydraulic pump) is damaged, causing serious internal leakage;
(5) Improper pressure adjustment of transmission pressure regulating valve;
(6) The gear-shifting pressure valve spring fails or breaks.