Full load issue in Pak128-Britain Extended

[TransportFanMar]

In 1750, I connected a vegetable farm and a market with a horse with capacity for 2 piece goods, the lowest possible. I then sent the horse to go back and forth between them, with a full load order at the farm. The first good filled up but the second one never filled up in 3-4 months despite there being demand (at least after a few months) and the storage of the farm being full. Why? Do I need to transport workers to the farm?

Edit: sometimes it shows when it will fill up accurately but why not all the time?

Edit 2: I am not sure 100% but think when it did that the vehicle remained half full and left despite the order!