My dieffenbachia is very healthy looking but it,s latest leaf will not open fully. It has been weeks now since it grew and opened half way. When I first got it, it got several new leaves quite quickly but seems to have stopped growing.
For me, sometimes dieffenbachia leaves get "stuck" and don't completely unfurl. Sometimes the top of a leaf will end up ripping because it was stuck. I think it's pretty normal. As long as the plant seems healthy, I wouldn't worry about slowing growth. You never know what conditions it was raised/shipped in, and it might be delayed stress from when you brought it home.