If a person, especially a child, acts up, they behave badly.

Sophie got bored and started acting up. 苏菲感到没意思，就开始调皮捣蛋起来。

If a machine or part of the body acts up, it does not perform as well as it should.

My car always acts up in cold weather. 天冷时，我的车老出故障。

Her shoulder was acting up (= hurting because of injury). 她的肩伤发作。