I assume that you're refering to the PST? The PST is somewhat similar to the Integrated Reasoning (IR) section of the GMAT. I would also recommend looking at strategies for the Critical Reasoning (CR) section of the GMAT - there are some good approached to identifying and then tackling questions designed to test your logical thinking.
I struggled with the same thing.
I would strongly recomment you practice it with the Integrated Reasoning part of the GMAT exam.
There are free exams in the internet that you can use for practice (the one of LBS MBA page, Verits prep, as well as some free trials for courses such as the one of The Economist (https://gmat.economist.com/)
Hope it helps!
Unfortunately, you've spent all the cases:(
Usually, I suggest the following:
Do one full case from the mck website in 60 min. Check your score. Target score is 22 out of 26 correct. If you do 17-18 correct you have great chances to improve it quite fast. If lower - it will take a couple of months
If your score is high:
If your score is low you need fundamental prep: