Being bored for a prolonged period is a good sign that it is time to move on, but I do not think that this has anything to do with working on an open source project or not.
The phrase "there is nothing new in the world..." applies to software as easily as it does to movies or books. Open or closed source, finding something new to do is a relative question. What I mean by this is: if I am writing a website for a clothing company using dotCMS, am I doing something new?
No: dotCMS (an open source CMS) is not new.
No: plenty of clothing companies have websites.
No: I am probably not going to implement new features in dotCMS as part of the project.
But more importantly:
YES: it is a new project for me and I will hopefully learn a thing or two.
NO: this is the fifteenth dotCMS project in a row and I am bored.
I think the above answers will be exactly the same whether you are talking about using open or closed source software.
In every job I have taken, I have been doing new things on previously established frameworks, APIs etc. They were open source, but that isn't the point. What I was doing was new to me.. so if you are bored, look around for something that is new. :)