I wonder how does promotion works in civil service.

Have been getting C for a consecutive last 3 years... But my peers who have been getting C is promoting faster than me...

My cep is also raised as confirmed by my boss. Current post in the dept is a div 1 position and they posted me (div 2) officer there cos my boss said he saw my potential... And that my cep has been raised (most likely to a div 1)

Wonder what went wrong.....