If you change the value of a textfield in the search toolbar (the one that has been integrated since version 3.7), you cannot execute the command triggerToolbar in any function called by the callback function of jqGrid. Only by putting the command inside a function called by a button or by using setTimeout().

More information can be found in this topic:

/blog/?page_id=393/help/add-value-to-filter-toolbar-then-trigger-the-reload-with-search/

p.s. see bottom of linked page for a current workaround.