There are a few reasons your raspberry cake filling might not be translucent.
One possible reason your raspberry cake filling is not translucent is that the raspberries were not fully ripe when you used them. Ripe raspberries are soft and have a bright, deep red color. If your raspberries were not fully ripe, they may have been too tart and not as sweet, which can affect the flavor and texture of your cake filling.
Another possibility is that you did not cook the filling long enough. The filling needs to be cooked until it is thick and bubbly in order to achieve a translucent look.