No bubbles when you first start using it?
Bubbles after a while and then you plunge it and no more bubbles?
Try using more water.
Solids tend to pile up at the bottom and eventually up the tube to the toilet. The sudden rush of "stuff" down the tube causes the trapped air up the tube causing your bubbles or burp.
More water in the tank will help those "solids" to breakdown and float away.
Of course, you could have the dreaded pyramid or clogged air vent, but if it acts as above probably not.