Esophagus not opened
If the esophagus it is not opened-up from your stomach up to your mouth, the gas won't come out.
Some spasm in any part of the length of your esophagus can occur and stop the gas from coming up.
Sometime it is easier to do than others...The more relaxed you are, the easier it is to burp...
Have fun with it, just keep it civil :-).