Spawn Chunk issue

When i build my iron farm, she didn't works when all of the player are disconnected, same problem when i cook potatoes, however when i put a commandblock who clear the time or an other commande, in the spawn chunk, he works perfectly, i can disconnect and see my terminal write "time set clear", so chunk is loaded for the command block but my farm didn't work and my potetoes didn't cooked :'(