I have a solution
Burn all outstanding docets. Abandon all the outstanding cases pending.
Now start of tomorrow with 0 crime and 0 investigations. Now deal with the new ones coming in as they happen and drive them thru till the end. The investigating officer will do nothing else till that case is resolved. If you need more people appoint more people.
That is the only way you will win. Currently they are so far behind they are lost and don't know where to touch what next. They have to investigate 40 - 50 cases at once

How the fk do you investigate even 2 cases at once ? You can't one case needs your full 100% of attention and nothing less
Now that is a solution