I think in any case, forcing your body to void the bladder and bowels isn't an easy thing to do. Your body, at least in my experience, tends to empty waste when in a relaxed state. That, and if you try to go to the bathroom when there isn't anything or very little to come out, then yeah you're gonna have a hard time.
For me personally, I had difficulty at first using diapers for their intended purpose. Often, I'd drink a whole bunch of water first, or if I had to go, I'd sit on the toilet with a diaper on to simulate going to the bathroom. Each time, it got a little easier, and I could then position myself as if I was on the toilet and go in any room in the house. Now, I can wet in just about any position with little force required. I've even done it while driving on a long road trip, and I sit fairly upright when I drive. I think it just comes down to a matter of familiarity and practice.