the less hair you have, the less irritation you get. so some techs think it’s better to get treated at lower settings and work their way up so that they cause less irritation. personally, my results are more important to me than 1 day irritation that goes away.
24 joules at 18mm are ok settings. If you can handle 26-28, great. But that would also mean that you could handle an alex laser for even better results, especially on the finer hair. Most of your problem is that you’re treating hair that’s not coarse enough as I said above (and the fact that your first 2-3 treatments were probably pretty ineffective at those settings, so if you technically count only the effective treatments, you’ve only had one and will still see results with 24 joules even at 18mm, but only on coarse hair).