The yellow only lasts a couple of days.
Try different brands. I've found the Carrs Billington own brand yellow and orange show up well and last longer than other makes such as Nettex Marksman.
Also some areas are more prone to rubbing off than others.
If you really need something long lasting then could you use raddle paste? There were traces in our tup's fleece for months after tupping. Just not sure if the oil would be an issue in the hotter months?