With any straw cup, I've found that I need to make sure that the "assembly" (for lack of a better word) needs to be put together tightly. If it's not, the straw comes very close to the bottom of the cup and sucking on the straw can actually create a vacuum. We've had this happen with BOTH Platex and Nuby cups and it always happens on airplanes (due to the low pressure). Unscrewing the lid and tightening everything up has always solved the problem for us.