Reasons for LOW COMPRESSION PRESSURE IN CYLINDER in HHP locos

LOW COMPRESSION PRESSURE IN CYLINDER 
a) IMPROPER EXHAUST VALVE TIMING ADJUSTMENT. 
b) IMPROPER EXHAUST VALVE SEATING. 
c) CRACK/ DAMAGE IN EXHAUST VALVE DISC. 
d) WORN OUT PISTON RINGS. 
e) EXHAUST VALVE BENT, DEFECTIVE CYLINDER HEADS. 
f) IMPROPER SEATING OF VALVE GUIDE.CRACKED PISTON.