I’ve read the posts & still can’t really tell if this chick is alone or has other chickadee friends...? One of the things I’ve learned on this site is that chickens are far from solitary animals, they are flock animals and even having just one other chicken to make a “flock” can have a very surprising effect on their ability to thrive. Also, I wonder about the heat source. I would think sitting fairly immobile on a heating pad in your lap may cause a vicious cycle. Constipation can be caused by being under active. Only having a small area for heat sourcing will keep the chick from being active, the way it would be in a warm brooder with the ability to move around without losing the heat source. Being so inactive can cause any appetite to decrease, while also keeping them at a slower metabolism rate.
Of course, this is only one opinion, one issue and very well may be miles off track from what is going on with your baby. Myself, I would be looking into getting at least one more chick, a brooder set up with food, water, heat and room enough for the chick to move around & be active.