Some folks just cannot carry AIWB successfully. It's one of those things that depend on a lot of factors to get right. You have to have the right body type (broad shoulders to waist ratio), wear the right size clothing, have a GOOD Gun Belt (stiff dual layered leather or nyon with a stiffener) and the right holster.
Having the ideal body type for appendix carry can be foregone if you have the best gear for the job. BUT the opposite is not true: you can be a body builder but still print while appendix carrying if your gear sucks.
Let me demonstrate:
Here is Jon. In the above picture, you can't tell that he's carrying a Glock 19 with 15+1 rounds at the appendix position. Why? Because he's 4 for 4 on the AIWB Scoreboard. He's got broad shoulders, his clothing fits how it's supposed to, he's wearing a dual layered Bullhide Gun Belt and he's wearing an AIWB specific Dara Holster. See below:
As you can see in the picture above, the holster is perfect for Appendix Carry. The ride height is high enough to get a good full purchase on the grip and also high enough not to jab you on the leg when sitting or bending. The belt clip stays perfectly aligned with the belt because it's the correct size. The belt is stiff enough to hold the fully loaded gun straight up so that the top of the gun doesn't flop out with gravity. Here's another shot:
Here is aside shot so you can see how much a good gun belt means in this carry position.
Scenario #2: Ideal body type, well fitting clothes, but bad belt.
Now, here is where the good holster part comes into play. You can have the right body type for appendix carry, wear the right sized clothing and have a good holster, but if you're wearing a bad belt- like the one you bought at Walmart 5 years ago- you're going to print like the guy on the left. And you'll most likely blame it on the holster.
Don't want to drop $80+ on a great gun belt? I hear ya, it's not cheap. What you can do- is get a holster that offers a wedge like the one below on the right. The Wedge is our integrated design that pushes the grip into the body. This let's you get away with not having a great belt.
The picture on the left is a standard AIWB Holster, the picture on the right has our Wedge option. Both photos were taken within 2 minutes of each other with a Glock 19 in the 12 o'clock position.