person67grepolis,
Either you have your Corporations set to "automatic" hiring. In which case they will reduce hiring if they have any unsold products.
Or you have a shortage of a particular type of workers. If even one type of employee type is in short supply, Hiring cannot reach 100%. Depending on which type of shortage you have. This is can be caused by either...
1). Your education index is to low.
2). Your education prioritises don't match your needs.
3). You have built more corporations than you have the workers to fully staff.
A more accurate answer could be provided, if you post the name of your country, and the world where it is based.
Signed President of DanNation on LU