Once below standard and restricted for selling, it is virtually impossible to improve except through passage of time.
I am not sure of the restrictions for a business account, but once your defect rate exceeds 2% you are below standard. To improve you therefore need a lot of sales which with a restriction is impossible.
Then as time passes old sales drop out of the equation but new sales are less so in fact your % gets worse.
For private sellers it can take as long as 12 months for all defects to drop out and get back to a satisfactory defect rate. As said the times may be different for Business Sellers, but the principle is the same, initially your defect rate gets worse, due to fall in sales.