jQuery Grid Plugin - jqGrid

Hello, I searched the forum but did not find any answer to this problem : I have a column with select formatter (data for the select are populated with a "setColProp" on beforeRequest event, using an ajax call - async: false - to a php script). When adding a record, all works fine with the defaultValue on this select, but only the first time after the grid is loaded, not after. Did someone ever encountered the same problem ? Thanks.