No chair—no matter how expensive or ergonomically correct it is supposed to be—will fit your back perfectly.
And if your back (both upper and lower) isn’t flush up against the back of your chair, then you aren’t getting the support you need to sit and type comfortably for eight plus hours a day.
Your back is your anchor—it needs to be stable so that it can support the rest of your body while you sit and type.
And because we are all built differently, all of us need something to fill in the inevitable gaps—however small—between our backs and the backs of our chairs.
I always suggest to clients that they use a small towel to fill the gaps. Just leave it on the back of your chair all the time, so that it is there when you need it. (It will also remind you to sit back—all the way back in your chair.)
It’s so easy to reach for the furniture catalogue and spend a few hundred (or more!) to try and solve the problem, but unless your chair is broken, it’s far better to spend some time working out where your back needs supporting.
One rolled up towel later and you, your back, and the rest of your body will be looking at a more comfortable future.