Sorry I have no idea why it was empty. This was about 1989 on the North Yorks Moors, so over 45 years since it may have been dropped.
Don't forget, dud IBs were almost 10 a penny after a raid. Very often parents made their children chuck their post-raid souvenirs away in the nearest pond or bury them at the end of the garden. I spoke to a former air raid warden who said there is a pond on Hampstead Heath which is supposedly full of them. After each raid they were told to dispose of the UX IBs in the pond.
I have often heard the rumour that dud IBs were due to slave labour who worked in the factories deliberately sabotaging the IBs by not filling them. I am sceptical as I have never seen any proof to back this up and think it may just be the fact that the fuzes in IBs were very basic - just a spring-loaded pin and percussion cap.
I have seen photos of the damage a SNIB did after it bounced off a parked Sherman tank - it left a right dent in the armour! More astoundingly, another photo showed the explosive power of the charge in a IBSEN - it left a 3ft x 1ft crater in a tarmac school playground. Then again, I suppose it was the equivalent to a hand grenade attached to an IB...