A shortage does not mean that the price will jump. It will start moving up and if it was very low, it may take some time before it reaches a profitable level. (3% per game month).
the same when it declines. It does not mean that profitability will disappear immediately. only longer term shortages will cause the price to decline to a much lower level.
There is nothing wrong with any of the types of corporations in Simcountry.