Hi Ros. 23 lb is nothing to sneeze at. That's a great loss for your first month following bariatric surgery.
If you think it should be more, or you think you can lose more, try keeping a food journal. Write down every single crumb you put in your mouth, and what time of day, and if you really want to, break it down to protein grams, calories, carbohydrates and portion size.
Also log your exercise if you walk or go to the gym. This will help you become very conscious of your movements and you can increase your activity when you want.
There's really no explanation for why people lose at different rates. It's just what happens. Are you pleased with your results, aside from the fact that you think you should have lost more?