Why won’t my butt plug stay inside me?

Hi I’m a gay man and since March I’ve started using a set of butt plugs (1″ diameter, 1.2″ diameter and 1.4″ diameter) they worked perfect until I started to loosen up so I ordered a new bigger 1.8″ and despite being able to keep it in all the way to the end. I tried using less lube but it still falls out. This is my butt plug:

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