1st two possibilities are over watering and or bad quality soil.
People are going to need more info like:
This is a paste, the questions are not specific to your situation.
The more information you give the better the help you will get.
What medium?
How often do you water and how much water do you use?
What nutrients are used and how often?
What temps and humidity?
Do you have fans for air circulation?
Stuff like that and other info will help.