You should really be allowed to click on a completely separate circle to start a new chain.
I was initially unsure about how the blockers worked, that should be made more clear. For others, if you create a chain out of a prime count of numbers, you may clear blockers. If you create a chain out of a composite count of numbers, you may create blockers.
Interesting game, but it doesn't have that same "I know I can do better" feel that 2048 grabbed everyone with :)