question

The drawing algorithm of the mass flow conveyor doesn't track individual items. It draws a number of flow units based on the density at a given point on the conveyor.

Here are some similar discussions on this topic:

https://answers.flexsim.com/questions/120295/flow-unit-traveling-is-not-visible-on-mass-flow-co.html

https://answers.flexsim.com/questions/135865/unable-to-set-number-of-units-in-mass-flow-conveyo.html

In your model, I advice you to activate show mass flow rate change points to show the lines that separate different densities on the conveyor:

Be aware that even if your source sends boxes to the mass flow conveyor, the entry transfer creates a number of flow units for each entering box:

https://docs.flexsim.com/en/23.0/Reference/PropertiesPanels/ConveyorPanels/MassFlowEntryTransfer/MassFlowEntryTransfer.html

And the size of flow units is defined in the toolbox:

https://docs.flexsim.com/en/23.0/Reference/Tools/ConveyorSystemTool/MassFlowUnits/MassFlowUnits.html

To better understand what happens, you can run the attached model massflowconveyorobjectinvisible_2.fsm.

Notice that when 100 flow units enter the mass flow conveyor, the drawing algorithm does not draw the flow units, very likely because the density is too low. But as soon as a second batch of 100 flow units accumulate with the first batch, the drawing algorithm decides that the density is now high enough to draw flow units.

Hope this helps.