This could be why they aren't eating much grower feed. If so, there is no need to worry. You might like to encourage this by planting things like clovers and other nutritious plants or allowing weeds to grow in some places (many weeds are very nutritious). There are things you can do to encourage bugs, too. An easy thing is leaving a pile of leaves. Or laying a board flat on the ground - you can turn it over after several days to let the chickens get at the bugs under it.
Free ranging gives many, many, many benefits.
If this is why they aren't eating not eating much of their grower feed then it is concerning. Mealworms and corn are both good feeds but only in moderation. If they are available all the time and lots of other options also are, the chickens will moderate how much they eat pretty well. But if they are given as treats, the chickens are more likely to eat to much of them. By "treats", I mean they are given with social activity like you call them over to eat them - verbally or through a "feeding time" routine.
Treating with mealworms and corn gives no benefits that treating with their grower feed doesn't also give (maybe mix some water into it to make a mash so it isn't exactly like their feed). They risk too much fat and too many calories for the amount of nutrition they provide.
You might also check that your grower feed is fresh.