Giuseppe's answer will likely correct the issue.
What can happen, STP can "block" a port when it first activates to avoid loops until it determines that it is safe to unblock the port.
Logically, I believe there are two cures. You inform the switch that the port can not be looped (which can be risky for user edge ports). Or, depending on the STP variant being used, you inform the switch to do the loop test much faster. (This latter, I believe, is what Giuseppe's recommendation does.)
