Q.New Leaves Won’t Open
I have 2 dieffenbachias – one is thick stemmed and has had to be cut and rooted again as it had reached the ceiling. The other is thin stemmed with much smaller, but healthy looking leaves, however any new leaves it sprouts remain rolled up (for months). I have just cut it to start new roots hoping this might help. These 2 plants are in the same area of the house and are watered ~ once a week, or if soil starts to feel dry. Both were also re-potted last year with the same new soil after a gnat problem (no pest issues since).

Certified GKH Gardening Expert
Try giving your plant more light (indirect but bright) and rotating a quarter turn regularly, and see if that helps. They are heavy feeders and need fertilizer every 4 to 6 weeks. Also, keep the soil moist but wait till the surface dries out between waterings.
https://www.gardeningknowhow.com/houseplants/dumb-cane/growing-dumbcane-dieffenbachia.htm