Tips: Make sure all the surfaces you are attaching the Velcro to are clean and dry. Also avoid any surfaces that have trays etc attached to them (that's why I attached the Velcro to the side of his wheelchair armrests rather than the top).
Update 24/09. I had a bit of issue with the stick on velcro coming off the wheelchair fabric straps. They needed washing, so I took them off, washed them and used IRON on velcro. Works perfectly and problem solved. If you can't take the straps off, maybe try a hair straightening iron. I haven't tried this but imagine it would work. Good luck.