struggling with polaroid emulsion lifts

hello! I have been attempting to do some polaroid emulsion lifts with some found polaroids before I switch over to taking my own- i eventually want to do transfers onto shells, etc, but am struggling with my preliminary tests. I have a book out from the library on polaroid image transfers and I know one of the key ways to do it is 15 seconds into its development time, but the book and some youtube videos describe an emulsion lift option for after the image is fully developed. The main problem I’m having is that the top plastic layer refuses to come off (and yes i’ve been soaking it in hot water). No matter what I do it doesn’t move at all. I have been able to get some somewhat cool effects with messing with the image on the plastic, but still can’t do the lift or transfer :( would appreciate any help thank you!