Technically, fly sparging should be a little more efficient if done to near perfection. Practically, at the home brewing level, there is generally little or no difference in the efficiency between batch and fly sparging. If you're not making expected OG your problem likely lies elsewhere. If you're brewing a kit or someone else's recipe don't expect to hit either's numbers. Differences in equipment and process will give different results.
Be sure you have set up an accurate equipment profile and adjusted your efficiency % to reflect your actual numbers then adjust the recipe as needed.