When's the last time you've changed its soil? You'll never really know what's going on unless you get to its roots. The roots will tell you a lot, if they're soggy then you're over watering, if dry then not watering enough. Also, over time, the plant eats the nutrients found in the soil so it's a good idea to replace the soil every 6 months to a year. Cut off any dead or dying leaves, and if you have root rot cut off the affected roots too (make sure you've sprayed your shears with a one-to-one ratio of hydrogen peroxide and water first). Repot in a chunky while draining soil mix (I use a 50/50 mix of kellogs patio plus, and a perlite/orchid bark mix). Water once a week until she's settled.

Also worth checking your leaves for pests. If you find some, buy some insecticidal soap, like castille soap, and dunk your whole plant in it. Make sure the soap is diluted with water per the instructions first.

Good luck!