Beginner Factorio Question - Inserters

https://preview.redd.it/g59jm9kympzg1.png?width=503&format=png&auto=webp&s=4417b2c39f88852a3299574b7941157b578bdef7

Hi - I'm new to Factorio, just a few days in and I'm having a frustrating issue in the Demo version of the Tutorial. I have created the main bus and branched off my iron plates to different crafting areas, but the inserters refuse to pick up the ingredients stating that there are no inputs. Here is a picture to show. Am I doing something wrong? It seems to be only the iron sheets that are causing the issue. The way I've fixed it is to delete the belt row and rebuild it and it runs for a few minutes before stopping again.