Prevent guards or obstacles
Your cat should be able to eat in a safe, non-threatening environment. This is a common issue in a multi-pet household. Whether you have multiple dogs or cats or a mix of both, having any pet guard the food may prevent the other from having their fill. Pay attention to this and prevent it from happening! Your cat should be able to eat in peace in order to gain weight.
Additionally, as mentioned above, your cat may not have liked the location of their bowl (especially if they are aging) or be hesitant to go near any disturbing element of the house (air conditioner, vacuum, noisy pipe, etc.). Be conscious of where you place their food bowl because it can make a big difference!