My advice is purely from lessons learned through my own experience and, in my plants’ best interest and mine, I check them regularly for early signs of distress, and acting quickly can go a long way to saving the plant. I have found that a struggling plant can have one or more factors determining its stress signals for help.
Firstly, check plants carefully before purchasing, as healthy plants are less likely to pick up problems. Be patient with your plant as it will need a few weeks to adjust to its new environment depending on the impact from the previous place to its new home. Quarantine you plant for the first 3-4 weeks will go a long way to prevent other plants from being infected with unwanted pests.
Tip: After a few unfortunate buys, I now give my new plants a good shower and mist the leaves and soil with a light preventative solution of insecticidal soap and filtered water and check it daily before placing it with my other plants. Happy plant, happy plant mom.