We’ve shipped some changes today to help clarify the state of your enqueued and concurrent builds.
As described in the docs, when your team’s concurrent build capacity is full, any additional builds will wait in your team queue, labeled as Enqueued: Awaiting Capacity on your team’s Builds page.
When one of your concurrent builds completes, we pull the next build in the queue to start building. Usually, one of our build machines will have a spot available to start that new build right away, flipping its label to Building. Sometimes, though, we’ll need to start up a new machine to run that next build. We’re now labeling those builds as Starting Up, until they reach the Building state.
We’ve also updated the Concurrent builds count at the top of the page to more accurately report the active and starting builds that count towards your team’s concurrent build capacity.
Big thanks to @kristy, @vivian, @charlie, and @nasivuela for designing and building these updates! 
As always, if you have any questions, feel free to ask!