Welcome to the forum. I agree with Jim. Scaling down the foam body size to match the hook size in the proper proportion or scaling up the hook size to match the foam body size seems to correct the problem and keep the hook down for me. Using enough tailing material so that when it gets wet, it holds the fly down on the water surface in an upright position is another possible solution for me if the fly proportion is right. Shortening the legging material might be another option if the fly does a "whirly bird" when casting it . Adding too much tailing material might also cause a fluttering effect. Best of luck.
Robert B. McCorquodale
"Flip a fly"