They can be very shy in some cases, they like shady hang out places below plants or hardscape. If you can direct flow into such a place they will sit there very happily fanning the flow.
They need a good amount of flow as I am sure you are already aware. I would avoid using fine filter floss in your system, so the water column will contain more particles for them to filter.
Target feeding them with a finely powdered food can be a good idea too, you can use a dropper or a syringe and release it gently in the flow in front of them 😊
Thank you for the food tip i will try to find where they have set up there base camp and look into feeding them directly. i have a basic filter which dosent use filter floss so thanks for that advice too