Could be any number of things ranging from:
Faulty injectors
High pressure fuel pump failure including regulator, split o ring etc
Low fuel pressure pump
Low fuel pressure sensor
High pressure fuel sensor
Need to carry out a structured diagnosis including monitoring live data from the engine management and voltages and testing the injectors
Faulty injectors
High pressure fuel pump failure including regulator, split o ring etc
Low fuel pressure pump
Low fuel pressure sensor
High pressure fuel sensor
Need to carry out a structured diagnosis including monitoring live data from the engine management and voltages and testing the injectors