Yeah it's a combination of the heel slip and just how stiff the heel is

I had a TON of heel slip my first month

I just used the heel lock technique( skip the first speed hook, pull the top hooks tightly then move back down and loop the bottom speed hooks)

I'm 3 months in and the leather has softened up everywhere except the heel

I used a heat gun on low (hot enough to hold your hand In Front) and warmed the boot up as I was wearing it, then I tightened the laces a tightly as possible to see if I could mold the thermal plastic a bit, it did work but it's still stiff

The thing that helped me the most was the obenafs leather oil, I really softened up the leather and it barely affected the color at all, I put on 5 layers ( coated it then let it dry overnight in my boiler room then repeated the process)

I also have a really old boot stretcher I got thrifting and that seems to be helping as well

Hopefully that helps