I don't think its possible to hit a weight lose plateau since the body constantly needs to be fueled by something. Maybe your body has adapted to how and when you do HIIT and isn't as effective as burning fat then.
Here's an article I found with a quick google search.
Weight loss plateau slow weight loss occurs why it stopped
I know what you mean by it can get discouraging, I'm going through the same ordeal; for awhile it seemed like it was going good and my fat burners were on hyperdrive and then all of a sudden they ran out of gas and that last amount I want to get rid of is superglued to my stomach. Just don't get discouraged or stressed man, thats probably the worst thing you could do, because with what little I do know, I know its hazardous to one's health. Everyone on here that I've had the pleasure of encountering is really helpful and everyone backs everyone here, its awesome. We all here for you bro.