My Tails monofin is a think slightly to big? Help?

SO this is my first tail. I bought a second hand Finfolk Mythic tail from another mermaid, we are similar in size. :cool: However I had a lot of trouble keeping my feet in the monofin. If i shoved my feet as far in the monofin as i could and got the comfort/power I was expecting. But after a short while my feet slipped back with my heels resting in the strap. :mad:I checked the strap ( kind of) to see if it was adjustable and so far concluded that it is not. I'm pretty sure it has feet pockets but the rubbed and rubbed and I couldn't swim very fast nor could I lift the tail easily. :jawdrop: So...obviously getting rid of the tail isnt an option. I Looked around and saw that perhaps socks would make it fit better? But am unsure. Any one else experience this issue? :phew:

Perhaps layer up on Neoprene socks?