Well, it depends on your goals first of all. What is the purpose behind uncovering a problem?
I sense it may be for the purpose of a personal project, for a portfolio perhaps?
I would suggest looking at the apps / websites / general things in your day to day life that you actually use and find trouble using (whether that may be a small issue or something significant). Start with that and I am sure you will uncover numerous problems from which you may choose a few problems you wish to solve.
Just my two cents.