This can happen if you added too much sugar, added the milk to the apple cider vinegar (instead of adding apple cider vinegar to the milk), added too much milk, or from not drying the strawberries after washing them. To fix this you can let the batter sit longer or add more flour, 2 tablespoons at a time. If it still doesn’t thicken you can still make waffles; just use about 2 to 4 tablespoons less batter per waffle when cooking.