In a Twitter post, Windows Insider head Dona Sarkar stated, 'We know not all of you are seeig (sic) the Fast Ring build. We're working on it." Sarkar did not offer a specific reason why this might be happening.
The new 14901 PC Insider build for Windows 10 is the first from the "Redstone 2" development branch, which will be the next major feature update for all Windows 10 users. It's reportedly due for release sometime in early 2017.
Update: Microsoft's support forums now have a thread discussing this issue, with the Windows 10 team asking Fast ring Insiders for assistance in solving this problem
Update 2: Dona Sarkar now believes the team has found the issue. She stated on Twitter, "we think we know what's up-related to the device pool. We are working on a fix, will update soon.
Update 3: Sarkar has posted again, stating that the new preview build should be showing up in the Fast ring