Why do some people experience pain on opiates or lack of focus on amphetamines? There is no simple answer, although, you don't specify the extent of your tolerance to this substance. Frankly, in my brief experience, tolerance to the cough-suppressing effect happens rather rapidly, after a small number of uses. It's also a possibility that you have a less than 100% pure source of N2O, and the additives contained in most whippits are rather potent lung irritants, in many cases. Add huge rips of cannabis to all of this and maybe you can start to see what the problem is, although you were rather vague in regards to your levels of use with these drugs, so I could be totally wrong.