The names Prisca, Sosipater, Silvanus, and Cephas are absent from Acts, but it is generally assumed that Acts mentions these people using slightly different names (Priscilla, Sopater, Silas, and Peter). Furthermore Luke and Lucius can be equated with the author of Acts.
Therefore, Acts does give a rather complete list of Paul's prominent companions. As far as I know the secondary literature cites only two exceptions to this rule: Titus and Stephanas. Titus was particularly prominent and the absence of this name from Acts has surprised many.
It is often tentatively suggested that Titus's involvement in the collection caused Luke to omit him. However, it is not clear why Luke would not just mention him anyway, while keeping silent about his role in the collection. Acts does mention the other collection delegates (Acts 20:4), so why would he omit Titus? Others suggest that Titus may have been a relative of Luke, and is omitted as a result of Luke's modesty. However, we have no evidence that Titus was related to Luke, and we have no evidence that conventions required that gospel writers omit the names of relatives. Indeed, early tradition (whether correct or not) has it that both Matthew and John named themselves in their gospels, and I have argued here the Luke/Lucius named himself in Acts. For more on the reason for the formal anonymity of the NT history books see A.D.Baum's paper in Nov Test 50(2008).
The anomalous cases of Titus and Stephanas are solved by supposing that (like Prisca, Sosipater, Silavanus and Cephas) they are mentioned in Acts under different names. I have already argued that Stephanas was Titius Justus. The absence of "Titus" from Acts raises the suspicion that he too appears there under a different name.
In my next post on the Titus-Timothy hypothesis I hope to present further evidence that "Titus" was not his only name.