I haven't used garden shades, but shed and fence treatments that leave a waxy, water-repellent coating can be difficult to recoat once they've dried. You can see how good they are by throwing a bucket of water at the surface. It the water beads up and runs off, it's good; if the surface is wetted, it's due another treatment.
I know two ways:
1) Wait a year or so for the surface to weather
2) Apply a fresh coat with a brush, rubbing it to and from along the grain until whatever the solvent is, softens the wax, and then you can recoat it because it has no longer "dried." On a hot sunny day this can happen even between coats.