I have a question t00. I've been alternating between running, exercise bike and HIIT for about 6 months now and it really works, has toned me up nicely but I never lose any weight. I don't diet but healthy eating is a way of life - I very rarely eat sweets or puddings - maybe a couple of times a month. My weight ranges between 10 stone 9 and 10 stone 11 , I'm 5 ft 6" and I've been a size 12 for about 30 years (apart from during pregnancy!), so I'm not fat but why do I weigh so much?
It's hard to tell without knowing how much you consume and what your getting into your body. It could be a number of things..
Sorry to ask but how old are you about?
Also are you doing anything else besides the cardio?
What exercises does your HIIT involve?
I can only suggest that your building muscle as that weighs heavier than fat, especially running as your building your glutes and your quads.. Largest muscles in your body.. your best bet is to take measurements!
Also keep an eye on your calorie intake, if you don't consume enough calories your metabolism slows right down. Whatever your burning off you have to put back in to your body.
Take a look at your diet are you consuming enough fat, carbs and protein?
Do you have any illnesses or are there any that run in your family?
A lot of these things can affect your weight loss, you also need to mix up your routine and shock your body. I'd say incorporate some weight exercises into your routine .. Ie squat to over head press.
That way you're working the larger muscle groups in with the smaller ones at the same time for maximum calorie burn