salesorder.getbykey works in one install but not another

I have a snippet of code wich works fine on most of the clients I install but one just doesnt seem to get a result

code is

success = salesorder.GetByKey(CInt(OrderNo))

where orderno = '1125'

I have confirmed the 1125 (or any order I choose) exist but I dont get the Sale order returned.

is there an index that needs to be updated orsomething like that

any pointer would be appreciated

thanks..

George S