While she is giving Ashley more attention

Ashley has been neglected her entire life and Mildred sympathizes with that. The extra care she gives Ashley is because she needs it so much more than her own daughters who are older, more independent and capable of holding their own. In comparison to Ashley who is too clumsy to handle any chores and has never had a mother figure or any other adult figure to guide her on how to behave. There is also the added fact that Mildred believes Ashley's happiness will lead to their happiness, but I honestly think it's secondary to Mildred's maternal instincts. We've seen that she focuses on her daughters based off their needs. If one doesn't need help she'll focus on the other ones that do. It's just that the person who often needs reassurance and care is Ashley. The closest metaphor is when someone new joins the class and Mildred is catching her up to speed. On the other hand, it must be jarring for her children to suddenly have to 'share' their mother. Their mother who had previously only given them her care and affection while neglecting Ashley. It understandable that they would perceive their mother as now favoring and prioritizing Ashley over themselves and feel resentful.

Mildred needs to sit down her children and explain that she loves and cares for them equally. That she would never prioritize one of them at the expense of another.

I don't think Mildred's ignorance to how her children have been feeling is as extreme as neglectful, they just need to communicate more.