It Took 60 Years To Realize Bilbo Had The One Ring
Throughout The Hobbit films, Gandalf never seems to buy into Bilbo's stories of just being lucky or his particularly miraculous feats – times when Bilbo was actually aided by the powers of the One Ring. It stands to reason that Gandalf pretty much knew Bilbo had a magic ring, though not the One Ring.
Still, it seems like a matter of paramount importance for Gandalf to know which magic ring Bilbo possessed not just for curiosity's sake, but for the safety of all Middle-earth. However, it's not until 60 years later when Bilbo vanishes from his 111th birthday party that Gandalf finally realizes the truth.
Given all the reasonable suspicions the wizard had during The Hobbit films for the well-being of Middle-earth, it seems like something the normally wise Gandalf should've uncovered much sooner.