If you're talking about the front axle bearings, the knuckles may be worn where the bearing presses in them. If the knuckles are worn, when you replace with new bearings, they won't last no time. The bearings should be a tight fit in the knuckles. If they go in without pressing them in, then the knuckles are worn and need replaced. That will cause the bearings to be loose even when newly replaced. It could also be that your axles are worn where they go through the bearings, but not likely. More than likely, your knuckles are worn.