Remember it takes one broadcast packet to get the loop started so if this a lab of just switches and you create the loop with no clients or ip's on you layer 2 vlans in the mix there is nothing on your layer 2 connections that would create a broadcast packet. you have to prime the pump ļ
I am not sure we have enough information to determine if a loop should happen or not. If you take a port with similar configuration and loop it into another port with the same configuration this should cause a loop as well. STP will not prevent a loop in this scenario unless some type of edge port configuration is used.