I always found brand new tubular tyres to be very hard to stretch on to a rim. I always put my foot in one end and slowly physically stretched them by pulling with my hands. I would immediately after try stretching then on to a rim without glue, just dry. You can usually manage after a bit to get it on the rim. Then inflate it hard and let it sit over night. Usually that does the trick for when you go to glue them on. They ride fabulously but I haven’t used them in twenty years because of all the typical issues.