That is perfectly normal.
Chickens dustbathe, preen, and take naps at some parts of the day.
They do this especially when there is hot sun, or cold wind, or when they see a predator.
I have watched on some hot days: when a cloud goes in front of the sun, the chickens come out of the bushes and eat grass on the lawn, and when the cloud moves away the chickens go right back into the shade to doze and preen and wait for the next "cool" stretch. I've seen similar behavior with chilly fall winds.
and free range only on a part of the area.
It's normal for free-range chickens to spend a lot of time in the areas with the most food, and the areas with the most shelter.
So for example, they might walk across a green lawn (eating a few bites of grass on the way), spend some time scratching under a bush (bugs and worms, cover), go to another bush (has ripe berries and cover), and then hang out in a shady spot for a few hours before heading off to find more food.
I want them to be more alive, find food and use the greater potential of the area.
Each chicken only needs so many maggots, or cicadas, or mulberries, or grass, or whatever other food each day. So they eat a bit of this and a bit of that, and it comes out sort-of balanced in the end. Also, some areas just don't have much food for a chicken, so they pay less attention to those areas.
(I feed them 2 times a day)
I would probably leave a commercial food available all the time. That would be chick starter, or grower feed, or layer feed or something of the sort, depending on the age and gender of your chickens.
I know different people have different experiences, but my own experience is that chickens prefer to forage for a lot of their food, and just munch a bit of the commercial food here and there, as long as I leave the commercial food available. But if the food shows up only for a short time, they try to eat as much as they can, as fast as they can, because they know it will run out. Yes, if you let them have it free choice after it has been limited, they will probably eat a lot for the first few days-- they do that with any "new" food, and then they settle down to eating a reasonable amount each day.