When the Emperor Claudius died, in October of 54 AD, Paul was at Ephesus. A teenage kid, Nero, was now officially Emperor. Perhaps unofficially, the more peculiar of Claudius' local decrees (Acts 18:2) became ignore-able. Thus, in Ephesus, Paul quickly realized that Jews could get back into Rome.
From the moment Paul heard Nero was ruling (not long after 1st Corinthians, and shortly before Acts 19:21), all his travel plans changed completely.
Unfortunately, poor Corinth had been waiting for a return visit from Paul since mid-52. In early 53, they'd heard about wisdom from Apollos. Later that same year they learned all about tongues and healing - and Jerusalem's three restrictions on gentiles - from Peter. The church also met Barnabas somehow, at least briefly, but Silas had the responsibility of two Macedonian churches AND Corinth.
Troubled Corinth, confused and divided, had sent letters to Paul around spring of 54, and Paul responded later that summer: having received further input from certain Corinthians, Paul sent Timothy and a letter. At that time, Paul planned to give Corinth only one more winter and spring before seeing his face. He would wait to sail, he promised them, until after Pentecost.
But then Claudius died, and Paul's promise was immediately postponed. From winter of 54/55 until winter of 56/57, among other things, Paul stayed busy by helping to coordinate a multi-church mission to plant a new church in Rome. Saints from Ephesus (Aquilla & Priscilla), Antioch (Rufus and his mother), and from other churches - everyone Paul names in Romans 16:5-15 - began making their various ways toward Rome. In 55/56, Paul and his co-workers even traveled as far as Illyricum (Dyrrrachium, at the western edge of the Via Egnatia) to plant a church (inscription found in Durres, Albania) as a half-way station.
It should be easy to imagine how doing all that wound up taking them about two whole years. Again, think about poor, struggling Corinth. They'd even lost Silas when Peter left, presumably visiting Paul before heading into North Asia Minor.
Before Paul left Ephesus for Macedonia/Illyricum, however, he asked Titus to visit Corinth on his behalf. Apparently, Titus went around Pentecost of 55, because Paul stayed in Ephesus a while longer, left after the riot, and expected to find Titus [back from Corinth] on his way north through Troas (2Cor.2:12-13).
Paul found Titus somewhere in Macedonia instead, and got a full report about the visit. From then, it was another year or more until late 56 when Paul wrote to Corinth again [Paul having gone down the length of the via Egnatia and back, while Timothy "stayed there in Ephesus", returned to find Timothy had arrived in Thessalonica, after leaving Ephesus to get help from Paul; in other words, Timothy's signature on 2nd Corinthians puts the letter after Illyricum, not before].
In that winter, late 56 AD - a year after Titus' report and fifty-something months since he'd set foot in southern Greece - Paul wrote a new letter to Corinth. From Macedonia, he sent a co-worker or two ahead with the letter (2Cor).
The state of Paul's concern for Corinth's well being makes more sense in this context. He really didn't know if the church would be (had been) able to hang together, or if it was about to (had already) fall(en) apart.
Coming soon: Did Corinth split? (the church in Cenchrea)