This is a general data example from an enterprise owned corp in one of my countries. Country tax rate is 75%, has been for a very long time. Welfare is around 90, it has remained close to that for a while also. Employment is slowing decreasing, but all corps are at 100% employment capacity.
Products Sold Current Month 3,207.55M SC$
Cash Available Now 41,585.34M SC$
Products Sold Last Year 39,262.44M SC$
Profit Last Year 6,730.21M SC$
Net Profit Last Year 1,682.55M SC$
Products Sold Last Month 3,245.32M SC$
Profit Last Month 446.91M SC$
Net Profit Last Month 111.73M SC$
Assets 90,171.01M SC$
Market Value 212,820.44M SC$
Outstanding Loans 0.00M SC$
Value of Supplies 15,063.99M SC$
Production Last Month 1,050,243.01 dropped down in one month from about one game year ago.
Production Level Last Month 107.70 %
A welfare index of 90 seem a bit low. Iirc, the welfare index affects how motivated your workers are. A low welfare index might be the reason production is off a bit. You might want to try and raise that to around 105-110 and see if that helps. Since only one month is showing, it is hard to spot the trend in the decline in taxes paid to your country from the corps. Therefore It is hard to see what else can be done to help. Taxes to fluctuate based on profits, so if the market supply and demand for that corp's product changes, sales and profits will fluctuate and as a result tax revenues will fluctuate as well.
The profits for each corp are gradually decreasing each month, at least for a year. I think it might be an increase in the cost of raw materials, can't be sure. What determines raw material usage, and what would make that cost gradually increase?
When you welfare index goes up, your workers will produce more product. This can increase the raw materials required to make the additional product. Also if the demand drops, producing the same amount could affect the price you get for each unit of production. With an extreme shortage of supply in relation to the demand, you can often get the max price. When the supply/demand gets closer to parity or the demand becomes lower than the supply, the price you get per unit will fall resulting in lower sales and consequently lower profits. Of course, raw material price fluctuations can also cause lower profits.