An Ordnance Decision Game
If the defective shells happened to be ones that could be used not just by French forces, but also by the enemy, then you could stealthily send tons of the defective shells across enemy lines, making it look like an accident and making the enemy think they stumbled on a weapons cache. Perhaps they would try to use the defective shells in their own guns and then boom! I’m sure this wouldn’t actually work for all kinds of reasons, but it was the first move that popped into my head and seems devious and comical.
1. Segregate from use all suspect lots of HE until they are inspected and approved.
2. Send inspection teams to the factories in question to review processes and ensure they meet standards. Make changes at the factories as needed and institute regular inspections at all facilities to ensure production standards are met.
3. Increase HE shell production at other factories not involved in this problem as much as possible to make up any differences.
Factory inspections. With attention to powder quality control. Something as simple as grinding the powder to fine could blow up a gun. At the scale of production I would guess excessive propellant dust is contaminating the bad lots in new factories.
You could stop using them as a forward fired projectile and use a new delivery system that makes use of the early detonation for top-down explosions.
To start with, I'd separate the ammo made at the new factories from the rest of the ammo.
I'd store this safely away from combat and then inspect those facilities.
Under no circumstances would I use ammo from the new factories until the problem is fixed.
In fact, I might switch those new factories to building the cannon and howitzer instead of the ammo.
In case the existing standards don't capture a manufacturing difference that makes a difference, do a detailed comparison of sample shells from the suspect lots to a similar sample of shells from the established factories. A comparison of the manufacturing processes and equipment used might also be fruitful.