As the subject says, one of our houseplant is struggling and we've run out of ideas on what to do to bring it back. We changed the pot to what you see in the photo, changed out the soil to a better potting mix containing aged pine bark, peat moss, perlite, vermiculite and a fertilizer blend. We water when necessary and give it adequate window light. Does anyone have any ideas on what else we can do? Thank you!

Short answer.. nope! Sometimes a plant gets too sick to recover. Next step is benign neglect. Depending your zone, water and place in a totally shady place. Humidity and temps are great for plants where I am in summer.

Hard to say what's causing it.

I'd trim off whatever's dead and dying, then do what sallyg said.

Normal for older growth to die off, but I'm seeing dead younger ones too.

Hard to tell but the soil looks very dry. Terra cotta pots will dry out much more quickly than plastic, especially under good sunlight. Love and prefer them though.

Removing what's yellow or brown now will let the roots stop wasting energy and use it for what's soon to come.

I used to love terra cotta.. in theory. I've gone almost 100% to plastic pots now. Because terra cotta dries so, and I tend to keep plants on the dry side.
I think that's your basic Dieffenbachia. I had one once, it died, one of very few plants that died on my mysteriously.
It looks like a good soil mix.

Thank you for the helpful information everyone. We will try your suggestions and see what happens.

I tried Dieffenbachia before, and it made me realize, it hated our dry area, even if I water it with care, I cannot provide the humidity it needs..sadly.
And I have seen them grow carefree and huge in my homeland, direct sun, part shade, shade..just happy there. Thinking about it, seems it likes high humidity and sustained warmth, nothing lower than 70F, temps may go higher provided humidity is high. And it gets such levels of rainfall there, so it is easily grown mostly outdoors.

I have seen some growers online where they enclose their humidity needy plants in a curio cabinet, where they can install a humidifier and fan. That seems to work for them..maybe think of ways you can do something similar. But do trim off all the dead leaves too, for aesthetics and help remove potential fungal issues.