This one may seem troubling but there’s a few methods to gain more objection situations. Just because you get a call that is objectionable doesn’t mean you’ll get it overturned either.
Play on Champion difficulty, as the CPU opponent will challenge you more and have you on the ropes. Influence the volley so that you’re right in the middle of the court. From there, hit a weak volley straight back to the CPU. This will give them an easy power shot to one of the corners. Since you’re in the middle, you’ll be able to lunge to either side and hit back. This cross court lunge shot tends to be close to the line, sometimes out. If it’s close you’ll be able to raise both hands in disgust and the chair umpire will review the call. Keep doing this until you get a shot overturned.
Alternatively, some favour working the volley so that you’re completely to one side, and attempting to hit up the line rather than cross court. Do what fits best for you.