And if it doesn't work on all pads, you may need an X-ray to figure it out.
Surface tension acts on all the solder balls, pulling the package into place along both the X and Y axes. So once the first few balls are in place, the whole thing “snaps” into alignment. Misaligning only some pads while others are perfectly aligned isn’t possible unless something’s seriously wrong, like a warped board or damaged package.
It’s an all-or-nothing situation—either everything aligns, or nothing does.
It's possible when your pcb-making process is on the odge of what designer wanted. Some pads on some boards may be not perfectly aligned in such cases. Or the board was heated not enough or in too short time and not all balls melted properly. Or it cracked under stresses because designer put too much vias in one place near the chip.
> It’s an all-or-nothing situation—either everything aligns, or nothing does.
In theory, practice and theory agrees. In practice, it sometimes does not.
I like to "wiggle" the chip with tweezers (under hot air) to see surface tension in action and judge whether the balls have all melted, but a tiny bit too much wiggle causes that problem.
Most of my soldering errors are from incorrect amount of solder, so, BGA tends to work better than other footprints.