Arnold, you may be looking in the wrong spot - That bearing is designed to snap into place within the truck - it may be your truck is damaged and not able to provide a latch needed for the bearing to "snap" into.
Look at the bearing and truck assy when they "fall out due to gravity", does the bearing actually fall out of the truck? Then look at the opposite end of the axle (presumably undamaged during the incident.) Does the bearing at the opposite end of the axle remain in place? Can you even just pull it out with your fingers? You shouldn't be able to on a normal part.
Long story short, if new bearings keep falling out, the truck assy is probably damaged.
The part shown here looks remarkably like the one you ordered - they both have a small tang or lip that is designed to snap into the truck assy - you can see it on the right backside of this part. Look at your truck where that part would snap into place, I'll bet it's broken.
Good luck! Hope you get it fixed!
George
