When plants germinate, they all start out with a certain amount of stored energy and nutrient reserves in their seeds. As those reserves are used up, the plant must find new nutrients from the soil in order to survive. Measure the gravity threshold for seedlings grown on the clinostat with and without extra nutrients. If not, try growing the seedlings for a longer period. The plants which weren't given any nutrients will begin to starve and have more trouble sensing and responding to the changing gravity on the clinostat.
Evidence suggests that plants can sense gravity within a few seconds of turning.
Instead, the plant can sense its constant state of tumbling but it can't respond fast enough to keep up with the rotation of the clinostat. For the fast clinostat, this means a plant's gravity sensing cells must not be located more than a few millimeters from the center of rotation or the plant will be able to sense a centripetal acceleration and the force it exerts. No one knows for sure. Once you have your clinostat running and you've had a chance to test a few batches of seedlings at one rotation rate, try doubling or halving the rate and see what happens. For rotations that are very slow, you should get plants that can both sense and respond to Earth's 1-g pull.