Ivermectin is great for external parasites but the studies show that it isn't very effective on internal parasites in poultry .If I were you I'd use a poultry wormer twice a year, such as wazine or rooster booster.
Free ranging is fine , there's nothing wrong with them picking their own grass as they pick it much shorter. Shavings could be your problem. I've lost chicks in the past due to ingesting them. Perhaps this hen is snacking on them., she may even be eating the sand , it tends to sit in the crop without being passed .Chickens will give anything a go, it's a process of elimination all you can do is tick all the boxes and do your best.
