I wish to give the user the chance to retrigger the latest search 

On your website I have found something like this: 

                  jQuery("#grid").jqGrid('setGridParam',  {

url:"dati.php?retriggerSearch=1&searchField="+searchField+"&searchString="+search_string,

 page:1}        )

.trigger("reloadGrid");  

The first time it works good. But when I go changing the parameters, they does not change anymore.

Better explain:

1 - first search all products are retrieved

2- search with built in jqgrid function for "red" color -- ok

3 - search with built in jqgrid function for "yellow" color -- ok

4- retrigger with above function red color , it always search for yellow color , as parameters from request are triggered differently

seems like the jqgrid is not modified with params

Thanks to all