Captain America might not have been able to lift Mjolnir in Avengers: Age of Ultron because of his relationship with Bucky 1 / 6
Steve intentionally hid his knowledge about Bucky being behind the death of Tony Stark's parents 2 / 6
This is after he found out about it during the events of Captain America: The Winter Soldier 3 / 6
He kept that secret for two years, including during the Age of Ultron party scene 4 / 6
It wasn't until Captain America: Civil War when he was confronted by Tony about it that he admitted 5 / 6
But with that secret now out, nothing's weighing his conscience anymore making him worthy of lifting the Mjolnir 6 / 6