I added my T2202 slip for my tuition and it decreased my tax return, why is that and what can I do to prevent this from happening?

This is correct to happen like this. The reason this happened is the Canada Workers Benefit (CWB) was automatically applied previously in your return due to low income, therefore, increasing your refund. When you included your tuition, the software detected that you were a student and removed the CWB as full time students are not usually eligible to receive the CWB. 

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