I would also rule out pain issues - and ask your vet about the possibility that she may have an ovarian tumour - I've attached a link that might help - or stomach ulcers due to the move and changes Equine ovarian tumors - DVM
The fact that you've moved her into a herd situation could be the problem. We all think of it as 'natural' so ideal when in fact for a horse not used to living like it it can be anything but natural and very stressful until they adjust and get accepted
The other thing to consider is that if a horse gets all it needs by way of stimulation from the herd it has less need for the human in its life so you have to make being with you better than being with them - but force feeding treats is not the answer - treats are OK to reward good behaviour or in a teaching situation but not as a bribe.
Man took horses out of the herd to make them better companions and work animals - turnout is important but has to be balanced with human contact and attention