I turned some ground this morning just to see if this did encourage them; they went and sat up a tree.

I think part of the problem is now they've decided that they like the commercial feed, as you point out, scratching for bugs when they already have a full stomach seems a bit pointless. It's a really stupid problem. For some it may not seem like a problem at all, but one of the reasons they tend to keep well and live a decent length of time I believe is due to the exercise and the varied diet they've had.
I'm trying the suggestion made by Perris and I'll see if they get out and about more this afternoon.