My lawn is green but 1/5 of my lawn is only moss. My house throws shadow on most of this 1/5 area with moss most of the year. Could that explain my problem?
To rule out the first two causes, you need to get your soil tested. Based on the results, the testing facility will make recommendations on what & how much to add.
If it is drainage or excessive shade that will require a different tact. Where are you located at (you may want to add this info to your profile)???