Not every navel is pierceable, just like not everyone can get an industrial, or a daith. This has nothing to do with weight or body type; it only depends on the shape of the navel itself, and how the tissue has settled and developed since the umbilical cord was cut.
Some navel shapes are not suitable for a “traditional” navel piercing but can support what we call a “floating” navel piercing; and some navel shapes are not pierceable at all, or the chances of rejection and scarring are significantly higher than the chances of the piercing healing successfully.
This is because some people’s navels fold in on themselves when they sit down or curl over. Or, the anatomy might be lacking that little ridge or “hood” of tissue that many people have at the top of their navel – this can put undue stress on a piercing and cause the jewellery to press forward and can lead to migration of the piercing, causing it to go crooked, and even as far as causing the piercing to reject entirely.
Some people’s navels are not able to support a traditional style navel piercing, but their navel is deep enough that a floating navel is the best option. Floating navels still use a curved barbell, but instead of a large gem on the bottom, I use a small disk or bead on the bottom that is often not visible from the front, and the top exit of the piercing is where a decorative end is threaded on. This style of piercing and jewellery keeps the piercing low profile on the bottom, so folks with wide but flat navels, or shallow navels, or very small navels can still have a piercing without a bulky piece on the bottom that, will not sit correctly, and worst case, will cause a great deal of pressure and trauma, leading to issues and possibly rejection or migration.