cork 5 is all about the set. Do you set with your legs open/shoulder width? If so, try carving in super hard instead. You need alot of speed and push your legs up like your jumping over a wall doing some parkour shit.
for me i learned corks by just carving into spins until it became hard to not land backseat, so i had to take it to cork. this was the easiest flip/cork trick i learned because i could carve into my spins. learn it with a carve, watch the jossi wells trick tip on it he does them pretty much how you should do it