Colibris have an internal recast that differs on spell for their reflect mechanic. If the recast isn't up when you cast on them, they won't cast it that time.

Cast a low tier nuke, wait for it to finish, then use the same nuke again, and it'll recast it it again.
Cast stun, wait for it to finish, then cast stun again (with fast cast/haste to reduce recast), and it won't recast stun the second time.