Situation
You positioned SmartSize's button next to (or inside) your variant picker. When a customer selects a variant (size, color etc.), SmartSize's button disappears.
Why this happens
This is issue is a combination of the store's theme and the location selected for SmartSize's button.
On some themes, when a variant (usually color or size) is selected, the entire container holding the size guide button reloads. Since SmartSize's size guide button is loaded when the page initially loads, it cannot reload within this container.
What to do
Recommended: move the size guide button
The simplest solution is to move the size guide button to outside the variant picker and into a stable area of your Product page.
The recommended way to do this is by using an App Block to position the button on your theme's product template.
Alternative: Enable dynamic content monitoring
If you must position the button in a place that gets changed during variant selections, you need to enable dynamic content monitoring by following these instructions: