On my back wheel on one side balls won't fit! I am 100% certain they are the same balls I took out easily with the magnet on my screwdriver several weeks ago to mail my bike home. They are 1/4" balls. I'm afraid if I get new balls, they wouldn't fit either since they are not worn any smaller. I even tried freezing the balls to shrink them and get them in, but that didn't work. Can I use 15/64" balls? They don't seem to fit the curve of the cone snugly anyway, but they do seem to fit the cup. I don't really want to take a ball out since it leaves such a large gap.
Are you sure you're putting in the right number of balls? Generally, when a bearing is new there is about a half-ball space left when you have a full complement of balls. As the bearing wears this gets to be closer to a full ball's space. It's easy to see the space and think you should put in another ball, but you don't need one.
(To hold the balls in place, of course, dab some grease into the race. Place the balls in the grease, and install the greased cone. This makes it fairly easy to reassemble a hub.)
It is extremely difficult to put even slightly worn ball bearing back into a hub. Over time they start to wear slightly unevenly, sometimes too slightly to be seen by the naked eye but slightly enough to make it difficult to line up again. You might need to take your wheel to an expert this time.