A character may have a disability, or they may acquire one (often as a plot-point), but that is generally it. Unless the story is about how their disability is cured, or about how they learn to live with it, or about how they are getting steadily worse, we don't see how a character's disability can vary from day to day, or how it may manifest in different ways, or how on some days they can do a wider or different range of activities to those they can do on others.
I remember Alison Bechdel's 'Dykes to Watch Out For' series featured a character who used a wheelchair on some days, and sticks on others, but that was so unusual that it's the only example I can come up with instantly nearly two decades later.
I'm not saying that other stories don't feature characters with disabilities that are well written, even though we need more of those characters, both as central characters and background characters where their disability is just one aspect that we are told about (in much the same way that we need a wider range of race and expressions of gender and sexuality in central and background characters). There are good portrayals of 'static' disabilities out there as well: Jean Taggart in Taggart springs to mind as a minor character in a long-running TV series, but I'm not so sure about progressive disabilities. I like Stephen Booth's novels, but I'm less sure about the way he gives one (perhaps two -- the second example is less clear) of his main characters a relative with a mental health issue that worsens in order to give that character more angst.
How about you? Where have you seen disability written well? What disabilities, or related issues, do we see rarely or not at all?