I suppose you have the problem when moving the conveyor with the controllers.
I made some tests to see what could be happening. My conclusion is that its highly probable that its something to do with the physiscs engine. Here you have a video that you can see the inconsistency in the way the objects interact with the standard ODE setting: Video Answer
Because depending on when and how the frictions work or they don't. Normally if you move the belt slow enough it will work because it will give the system time to make the friction calculations.
My recommendation is that you try different configurations in the ODE parameters in the world.
Also try different friction parameters to make the friction bigger.
But As I sais it seems more of a ODE system issue than the friction coeficients values.
You could for example try using the "world" solver type instead of the "quick". Its more precise. Also increase the itterations and the update rate.
I post here as much code as I can:
conveyor.urdf.xacro
main.launch
friction_test_object.urdf
conveyor.world
spawn_friction_test_object.launch
spawn_conveyor.launch
conveyor.yaml
conveyor_start_control.launch
