When I open the model in 14.2 there are 2 spots in the buffer (B_FORNO) and 4 spots in the oven (FORNO). Perhaps this is the reason it does not work as you expected.
I'm sorry but i've changed the oven capacity and it doesn't work as i would, again.
I would lock the buffer before the oven, in order to release at the same time this 2 MU's, that will be heated in the oven, at the same time.
I don't know how to change this behaviour.
What exactly is it that does not work? Please give a detailed description to the flow you want to model.
1. Products enter B_Forno
2. Products wait in B_Forno until Forno is empty
3. Now X products in B_Forno move into Forno . (should it be 2 or 4? Since you have 2 spots in buffer, but 4 in Forno you can't fill upp Forno with this logic...)