To increase your welfare index in the country, health, education, transportations are OK at a level of 120-130.
A higher level of these indexes does not influence the welfare index.
Also, employment should be reasonable (85+), supplies should be optimal (no shortages),
and the social security index at 80+.
If you have sold population in recent (real) months, it will have a very bad influence. to fix it quickly, you need to purchase population. You can also wait, but it will take time to repair.
a very low quality of products in the country (purchasing level), can cost up to 3 points.
The index is influenced by the lowest values of these indexes, it is not an average. a supply index of 20 could alone, crash the welfare index.
a welfare index of 122 is excellent.