dynamic text not changing values

Hi all

I have an if then else condition for dynamic test as follows.

if {Sales} >5000 then "Good" else if {Sales}>4000 then "OK" else "Low"

It gets it right the first time but does not change after that. For example: if I filter to get Sales value of 4300, it writes "OK", but on changing the filter back to 6000, it does not change value. And if my first value is 6000, it gives "Good" and never changes that value no matter how my values change.  Has somebody else also faced the same issue? Any help will be appreciated.

Bala