A single ICV injection of streptozotocin (STZ) has a mortality rate of around 13%. A second STZ injection increases the mortality rate to 25%.
Article Streptozotocin as a tool for induction of rat models of diab...
https://pubmed.ncbi.nlm.nih.gov/22886014/
The only thing you can do is to have more mice available for your experiments. Double check your injection rate and the amount you are injecting to prevent death by mechanical stress.