Light radiographic film is due to:
LIGHT RADIOGRAPHS 
Processing Errors 
Underdevelopment (temperature too low; time too short; thermometer inaccurate) Depleted developer solution 
Diluted or contaminated developer 
Excessive fixation 
Underexposure 
Insufficient mA 
Insufficient kVp 
Insufficient time 
Film-source distance too great 
Film packet reversed in mouth 
DARK RADIOGRAPHS
Processing Errors 
Overdevelopment (temperature too high; time too long) 
Developer concentration too high 
Inadequate time in fixer
Accidental exposure to light 
Improper safelighting 
Storage of films without shielding, at too high a temperature, or past expiration date
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 80
Overdeveloping
Overfixing
Directly putting in fixer
Exposure to sunlight