the most common problem people have with houseplants is overwatering them. just because they look bad doesn’t necessarily mean they need water. even though most houseplants are tropical in origin (were they are used to lots of water), a potted plant doesn’t have the drainage a plant in the ground does.
some plants just go dormant for a while-especially this time of year.
one should not fertilize plants during their dormancy or after transplanting either.
try looking at the plants location. some like direct sun, some like indirect sun, some like shade.
test your soil. if it’s hard and compacted try repotting them with new soil.
test your soil ph. some like it basic, some acidic.
research your specific types on google.
also, no leaf lasts forever. plants lose their leaves naturally each year or so.
don’t just jump to watering to fix a sick plant because you might just be overwatering and will kill it.
different plants like different conditions of sun, soil type, nutrients, ph, and temperature.
read some books or google houseplants.
best of luck.