There really are only a few basic things that cause a plants health to diminish. These are just a few off the top of my head.
1. Heat - Plants getting to hot or the lack of heat.
2. Ph - Ph balance out of wack.
3. Nutes - To heavy on the nutes, or no nutes at all.
4. Water - Over watering, or not watering at all.
5. Bugs - Bugs or parasites eating at roots or leaves.
6. Lights - Over exposure or under exposure.
7. Soil - Hard and or compacted soil.
8. Minerals - To hard of minerals and or metals.
9. Hours - Hours the plant has been under light. Typical for plants to droop towards end of the plants day. Some plants also can't handle a 24/7 lighting (some).
If anyone else can do any better or see that I have forgotten something, feel free to speak up!