I hate to have to point these things out as a fan, but...
*Her public image is still too childish/Disney to sell very well. To people who point to Miley to say that I'm wrong, note that Miley obvs has a more varied audience now (kids, 14-20 year old tumblr kids, older buyers getting the album for their kids, etc.) while Demi mainly just has a smaller crowd of kids, teen girls, and gays.
*Demi/her image is not very memorable or distictive, which is very important. Almost every pop star needs some sort of image in order to be recognized by the public (Gaga has the odd art student image, Bey has her fierce glamazonian Goddess image, Miley has the "ratchet" twerking weed-smoking tumblr swag image, etc). Some musical growth, better music videos, etc. can fix that
*Her label is kinda ****. She can have multiple hits (Demi itself has 5-7 commercially viable songs), but her label doesn't know how to promote and when to send songs to radio and stuff like that.
She + Selena need to both leave Hollywood Records tbh. It's good for growing Disney artists like Ariana and Zendaya, but you can't base more than 4-7 years of your career on it and expect to be in the big leagues imo. I would love to see Demi do Breakaway/The Truth About Love-eqsue pop rock for her next album. It's stuff that seems to be in her niche, but would still do well. idk