Donald Trump,
Corporations value declines when they are loosing money. One common cause of failing corps is when they are unable to hire all the workers they need. The main cause of this, is if you built too many Corps for your population, or if your educated system is failing to produce enough educated workers.
If you don't have a lot of unemployed workers, then your problem is that you have built too many corps. Closing some of the failing Corps will solve this problem.
If you do have a lot of unemployed workers, but a shortage of Managers or High Tech Workers. Then your problem is your education system. If you are struggling to understand your education system, I suggest you turn on automation for it, before your country gets into deeper trouble.
Turning on Education Automation can be done by following these steps
1) Select your Nation
2) Click the "Automation" button, near the top of the page.
3) There will be a tick box menu, make sure AT LEAST the following items are ticked
3.1) "Economic Support - Easy Mode"
3.2) "Education setup and development"
3.3) "Move Workers to other Professions"
4) Once you have ticked the relevant boxes, press the button "Save Automation Features", located towards the bottom of the page.
These settings will keep your Education system reasonably balanced, while you learn to understand it.
Other things that can help Corp Profitability,
1) Don't overpay your workers, if your Corp is paying +300 wage index, you're paying to much (unless your welfare index is high, then it can be profitable to pay high wages)
2) Control the Quality of the goods your Corps buy, a value between 120-220 is ok, anything higher than 220 is bad. 120 Is best if you haven't fully upgraded your Corp yet, otherwise 180-220 is better.
3) Make sure your buying and selling the goods you produce at good prices, "Best Price" is the best option for people who don't fully understand buying and selling strategy
4) Fully upgrade your Corps, high level Corps are generally more profitable than base level Corps.
5) Improve your Welfare Index, this can improve Corp productivity
6) Don't overtax your Corps, any tax rate above 30% is probably too high
Also, if you provide the name of your nation. People might be able to offer more personalised advice/assistance. I hope these suggestions can help.
Signed President of DanNation on LU