1. Brooding Cover
2. Nesting Cover
3. Roosting Cover
5. Food
4. Escape Cover from aerials

Farming practice fence to fence is a big part of problem.
Loss of cultivated land /food source is a big problem in my opinion.
60 to 80% of some counties went into CRP grass in the 1980s and these lands have turned barren to quail for food source.
Get your land owner to talk to ASC about the problem and manage the land for quail. Most just let it set in grass with no seed crop for quail. There are several financial program to help with management of lands. And the CRP can be broken up into strips of native grass and food plots for wildlife. They will even pay you for the set asid strips and planting in other benefical grasses.

Quail will abandon over-mature CRP fields
When the grass begins to crowd out the legumes and annual seed producing plants (usually 3 to 4 years after establishment), the main benefit is nesting cover. Once the legumes and patches of bare ground disappear, quail will seldom use the field. To keep quail around, manage CRP field to maintain their habitat needs.

When you take away 80% of the food source they are gone. BTDT an seen it over and over.