I have studied different conveyor assembly line and came to realize that when entities are getting assembled on a conveyor assembly line, it never forms buffer on the line. Parts can not move to other station untill its predecessor or successor doesn't accomplish its task completely.
if you are talking of one long conveyor, I think that is the natural behaviour.
If you want the parts to buffer, then model an individual conveyor between 2 stations
The Line object provides the required settings to model most real conveyor line configurations. The most usefull ones are: Accumulating, Speed. For the MUs you can use the Stopped attribute to model a hard stopper at a workstation. If you put this in a sensor control and combine this with Accumulating=true, then you cna use a lingle Line object for a longer conveyor stretch.
You need to take into account the transport-system which is used. In many cases these are chain-conveyors and this type of equipment is not able to create buffer (it is not accumulating). In many cases the conveyor is moving (very slow) while the worker assemble parts. Here you have the concept of over-cycling to cover different assembly times (e.g. the working area is 6 meters, the normal process takes only 5 meters, 1 meter is available for over-cycling).
But in each case you have to look at the technical kind of line.
freelance simulation specialist