Some horses have that "phlegmatic" personality; nothing seems to get them excited. In some cases, it's because they are stuffing their anxiety down inside and appear to be unreactive to things, just not moving, but when the stiumuls gets beyond their ability to stay frozen, they just come apart, like a bomb.
However, from your description, your horse strikes me as the more dull variety, not the stuffing it in kind. As to building softness )aka responsiveness), all I can think of is that you have to really raise your expectations and abide by them until she meets them.
So, for example, under saddle if you ask for a halt, you apply the pressure to the reins (and I think it helps to have one rein be a bit firmer than the other) and you apply it slowly but you give her only three steps before your hands and body become concrete!. She will run into the brick wall of the bit which you will not give an inch until she does. In the beginning, you require her to only give a little, but as you go, you keep the brick wall on that bit until she really gives her whole head at the poll and releases hersefl from the pressure of the bit, then you also give her a lot of release. As you go along doing this, you sloly raise the levle of your expectaiton.
So, start out asking for a stop within 3 to 4 steps and work into one step (thoug you MUST give her a signal with your seat and body that you are asking for a stop before you apply the reins . . . Always. If you don't, it's not fair.)
Keep her to the standard and review frequently. Don't accept sluggish and half hearted tries on her part, once she understand the idea of feeling the bit halt and her kind of bouncing off that bit. Back her up even.
In general, you require more of her . She can easily dull you out with that kind of behavior, so you have to stay sharp yourself.