Some trees will give them the runs, i think Ash gave mine problems, or have you turned them on to some fresh, rich grass? Or if they aren't eating the hay before turning out, wet grass could give them runs.
I have always believed you should leave new hay for 6 weeks before using. Do you know any horsey people who are feeding good haylage? Might be worth seeing if you can buy a bit for a few weeks. Mine preferred haylage during winter, but it was nice dryish stuff. Don't use it fast enough during summer.
Mine get a mix of ewe and beef nuts, milker gets ewe and dairy nuts, sometimes goat mix, sometimes ewe mix, i likebana to give them a bit of a change, the kids seem to prefer beef nuts to goat Mix!