One frequent problem that occurs when intubating over a bougie is that you might get caught with the tip of the endotracheal tube (ETT) in the epiglottic area and therefore can't advance into the trachea.
When you look at the ETT the way it is fabricated you will notice, that the distal opening is shaped angularly and the foremost tip of the tube is on the right. This tip has a good chance of getting caught at one of the structures in the epiglottic region.
The simplest solution is to retract the tube a little and perform an anti-clockwise turn.
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