Answer: The unknown plant in the last photo is a Dracaena.
The dieffenbachia looks like it may have several problems. One, it would probably do better with increased indirect light. These plants are known to be tolerant of low light, but they do not stay healthy in dim situations such as can occur in the winter months. You would also want to rotate the plant every week or so to keep it growing straighter. Leaning toward the light can cause the plant to become unbalanced and also weak. Low light will also cause the plant to lose foliage.
In one shot it almost looks like there may be a bit of fuzz on the leaves. That, along with the discoloration, looks like there might be a possible spider mite infestation. These cause fine webbing along the stem and underside of the leaves, the leaves discolor and drop off... Rinsing with tepid water once a week and raising the ambient humidity can help control them, as can spraying with commercially formulated insecticidal soap per the label directions, be sure to apply and repply as directed.
Next, it may be overwatered. This can happen in winter when the plant grows very little and uses less water as a result. Overwatering and constantly cold, wet soil can cause root damage and foliage loss, typically from the bottom of the plant moving upward. It can also lead to a stem tipping over.
In one shot the bent stem has a sidebranch growing almost straight up. That little branch looked pretty healthy in the picture. The older sideways stem might have been able to root into the potting mix where it touched the surface just beneath that upright growth tip. In other words, this would have resulted in a natural case of layering with new roots growing to support that healthy growing tip.
Lastly, this is a tropical plant that does not do well in cold temperatures. If there is a cold draft by the window, for example, or if it is sitting in a cooler room and down on the floor, it may just be too cold for it. Try to give it a jungle atmosphere, by that I mean about 75 degrees and humid.
I hope this helps you trouble shoot.
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