Vinod
The trigger cell is a concatenation of multiple cell which means for every change in the individual cell, the trigger cell changes and the query will be triggered.
Have you tried displaying B15,16,17,18 individually and check if more than one cell is changing value?
If so, you will have to use the option When becomes and add another component to the concatenation to make sure the entire trigger value matches only after all the individual cells have been updated.