I find you have to hoik your weight back a lot more.
When I was intro BMX I could manual for ages, hop up things, jump things etc. all off the back wheel. Go up to a 24 and it is harder: XC26er and, for me, almost impossible.
I have a Karate Monkey now, with a 29+ tyre on the front and I can at least get the front up to get over stuff, but wheelying or manualing for any decent amount of time is just beyond me. I think the more weight you have in front of the rear axle, added to the larger circumference of the wheels, makes it a lot harder to get that front up.
Maybe try a few sessions practicing wheelies and manuals, just to see where the balance point is and how much you need to muscle the front wheel up.