Answer: When you bring a plant home, it will take it several weeks to become acclimated to the growing conditions in your home. During that time it goes through a transitional period or adjustment period while it gets used to the light and humidity in your house as opposed to at the commercial growers.
For the best results, you need to select a plant that is well suited to the lighting conditions where it will be placed. Then allow the plant a couple of weeks to settle into its new location.
At that point, if it needs repotting, you could do that. Repotting is also a big transition for the plant. To minimize the chance of shocking it, you need to use a soil mix that is well suited to the plant and also a fairly close match to the soil it is already potted in. Move to only a slightly larger container. When you repot, tease any potbound roots loose so they can grow into the new soil.
Take care not to overwater the newly repotted plant, it's best to use your finger to feel both the old and new soil to check and see if you need to water. Probably the biggest cause for loss of houseplants is over watering. Be sure to water according to the specific plant's preference -- some need to dry out between waterings, some need evenly moist (damp like a wrung out sponge but not sopping wet/saturated) soil.
Overfertilizing can also stress a plant. If the new soil includes fertilizer, you would not need to fertilize in addition to that for several months. If in doubt, it is better to underfertilize than over apply it. When you do fertilize, read and follow the label directions for how much to use. In general, water soluble fertilizer and slow release granular fertilizers such as 10-10-10 plus minors are fine to use.
I hope this helps you trouble shoot.
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