Thanks for the tips guys.

Overspray of poison is not the issue and I doubt over watering because
the garden was often dry.

No relationship with any gardening neighbours to ask them and now I
have a new problem... my new chickens (only got em in the last 2 weeks
so they're not the culprits) jump my fence and dig up the ground.

Might have to give up on the project.

Sad, because they seemed to sprout so well on my balcony.