What’s going on with my Dieffenbachia? 😣

My Dieffenbachia isn’t looking as good as it used to (second picture is from May last year). The variegation is almost completely gone, and while the leaves are firm, they look a bit odd.

It’s always been in a north-facing windowsill with bright indirect light and only gets about an hour of direct sunlight in the summer. I water it when the top 2-3 inches of soil feel dry.

Lately, its growth seems stunted—new leaves aren’t growing like they used to. This is actually my third attempt with this Dieffenbachia, from my third propagation. They always start off with beautiful, variegated leaves, but after a while, things start going wrong. I haven’t checked the roots since the soil is quite damp, but I don’t think it’s rootbound as there are no roots visible in the drainage hole.

Any advice would be appreciated!