It's probably for two reasons. The first reason being that the Durant are a higher LV than you. The other reason is probably they are faster than your first Pokemon in your party Speed wise. Hope this helps...
There are lots of different reasons. One could be it is a higher level than every other pokemon in your party. Others are it has higher stats then other pokemon in your party, or that Durants are uneffected by Max Repels. (Other strong pokemon, such as Gyarados, are as well)
Is your pokemon's level higher then them? if so, the repel won't work...
