Boots/converse folding at the heel

[petridishmystery]

Hoping someone can help me or at least give me a better search term! I have some sort of canvas type ankle boots and also today bought some Converse. They both seem to sort of fold at the back, where my heel is, rather than staying straight up. Tried googling it but just brings up styles of folded down boots! Does anyone get what I mean from that rubbish description? Obviously it means they don’t fit right but I’m not sure what part of the fit is wrong as they don’t feel like they’re too big but then the converse end up sort of flapping! Not that I can wear them anyway as the back is so stiff it’s rubbing on the back of my ankles.

It sounds more like they're too small

I can pull them on ok and there’s a little bit of room when I’m just stood still, it’s as soon as I try to walk they fold

I think they sound either too small or too big but unsure which.
For example if you have wide-ish feet and the boot is slim fitting then you'd need a larger size for the width but the boot would then be too long in the length which is why it's folding?
I know someone that had some and they were slightly too small and they did this too