15Gandalf's Short Term Memory Loss
The blame here lies in The Hobbit trilogy rather than anything in the Lord of the Rings. Still the cinematic version of The Hobbit has retroactively made Gandalf into a bit of dummy during Fellowship of the Ring.
In an effort to pad out the prequel story to
three movies, a lot of subplots were added to Bilbo’s adventures, with several characters getting long arcs that exist nowhere else in the lore.
One of these additions is Gandalf being put on the trail of the One Ring far earlier than intended. Throughout The Hobbit trilogy, Gandalf figures out or at the very least strongly suspects that Sauron’s return is imminent. Yet by Fellowship, he's forgotten all about it with no explanation.
Even if it's decades later that's still a blink of an eye for Gandalf. Although in The Battle of the Five Armies, Sauron is “banished” Gandalf is still unreasonably shocked to find out the truth about the Dark Lord and the ring in Fellowship of the Ring.
Based on The Hobbit it should Gandalf an hour, tops, to figure the truth of Bilbo’s prized possession but instead it takes months.