It sounds like you are trying to use decision points to define a section of your conveyor system where only one item should cross at a time.

Here are two example of how to do that:
Acquire and Release Area Pickoptions in On Arrival Triggers
This model uses the Acquire Area pickoption in the On Arrival triggers of decision points that represent the start of the restricted area and the Release Area pickoption in the On Arrival triggers of the decision points that represent the end of the restricted area.
You can find more information on these pickoptions in Help > User Manual > Conveyors > Pick List Behaviors > Area Restriction.

Process Flow with a Zone
This model uses Process Flow to listen to the On Arrival of the various decision points. It uses a Zone to restrict access to one item at a time.

Thanks @regan.blackett for the models!
Matthew Gillespie
FlexSim Software Developer