Proper Substitution Rule?

[officialtony]

I am the Referee in a JV match. Team B attempts to substitute 35 for 27. Umpire signals illegal substitution and loss of rally/point. At the end of the match, I asked what he had on the call. He said 35 tried to enter for 27 who had already replaced starter 33. He told the coach that the only person who could re-enter for 27 was the starter 33. Is this correct? I thought anyone could enter into that position in the rotation as long as they had not entered into any other position during this particular game. Is their a specific rule stating who can enter in as a replacement? Please help.