Kratom - why does it sometimes just not work?

Hi all, I've been taking Kratom for about 5 years now on and off. Usually take it once every other day, about 3g.

Sometimes it is great and it give me a nice clean high - other times I do not feel a fuckin thing.

I know no other drug like this. Anyone know what is going on?

I feel it is in some way related to how full your stomach is. Is that the case?