Abs not feeling the burn

Hello!

I'm not new to working out, and have been exercising fairly consistently for about 2 years now. I recently started FB Blend (my first Fitness Blender program) and have just completed Day 10, the second Abs + HIIT day.

I've now done two of the program's Abs + HIIT days and am feeling kind of unsatisfied after them. During the exercises I am giving them my all, keeping an eye on my form (I am not new to working out as I've mentioned, so I feel I have a pretty good sense of when my form is off) but I am not feeling a "burning" sensation in my abs during the workout. I feel a general sort of tired (not specifically that my core is burnt out), work up a good sweat, and some exercises are a struggle, but as much as I focus on activating my core and syncing my breath, I'm not feeling that burn!

I have definitely felt a burn in my abs while working on my core before (as recently as a couple of months ago if I had to guess) but now it just isn't happening... Last week I waited to see if I would feel soreness the next day, and I did feel a bit of soreness in my core, but just a bit in my upper abs and obliques, and absolutely nothing in my lower abs. I know soreness isn't a good measure of exertion necessarily, but I was hoping that since my effort wasn't paying off in a burn during the workout, that maybe it would show up as DOMS, but it didn't really.

I don't think it's to do specifically with FB, as I tried returning to the kind of workout where I used to feel the burn in my abs (a very mindful and activation focused yoga video) and though I exerted myself to a point where my torso was SHAKING, I still didn't feel that signature burn.

So does anyone have any guesses as to what might be up and what I might be doing wrong? I'm not dead set on feeling the burn, but does not feeling it mean I'm not working to my fullest? I wouldn't be so fixated if this wasn't something I had experienced before and used as a guide for how much effort I was putting in. Now I'm not so sure if I'm working hard enough or wrong or what!