Clojurians Log v2

Why does this hang?

(let [p (m/stream (m/seed '[1 2 3 4]))]
  ((m/join vector
     (m/reduce conj p)
     (m/reduce conj p))
   prn prn))

It works if instead of m/seed I use (m/ap (m/? (m/sleep 1)) (m/amb 1 2 3 4))