Mistake : forgetting the base case. If you do not handle null nodes or leaves, your recursion breaks. Mistake : processing nodes before children. Use post-order (children first), not pre-order (parent first). The parent needs child values.
Mistake : confusing returned value with tracked answer. One goes to the parent, the other is your final result. If your tree DP fails, check these three things first. They are the most common errors.