Well, I didn't see the demo (is there a vid, BTW?), but the basics are fairly simple (much simpler if you've done all this before; not so simple if you're not used to connecting your body to your middle, but give it a try).

To over-simplify the setup, imagine that each of your arms have a big block of (non-cold) ice frozen around each one and your left arm (and ice block) is resting on a shoulder-level table on your left side and your right arm (and ice block) is resting on a shoulder-level table on your right side. IF your arm is extended and held in such a way that moving your middle is connected to the ice blocks, you can make the ice-blocks slide on the table without having to use the local arm/shoulder muscles. If there is a slackness (say a loose shoulder joint) between your middle and the ice-block, your middle will have no control over the ice-block on that side, right?

So what happened was that you didn't understand the principle, so you didn't really know what was expected of you. Uke (the loose one) didn't understand the idea, so he was providing a poor connection for you and thus you couldn't control his center with your center. And naturally there are angles in which it is easy to move uke (usually toward his back "gate" between the legs or the front gate) and angles in which it is almost impossible to move uke (like if you try to off-balance him in the direction of his supporting foot).

So Ikeda's demonstration sounds pretty valid and kokyu-correct to me, but if you and uke don't really understand how to do it then it can be frustrating. I've heard that Ikeda Sensei often says something indicating to "make one" with uke or "tension to uke", etc..... this is what it means. There has to be a solid connection between you and uke or you can't control his center with the movement (or intention) coming from your center. Also, it's easier to move an uke when he has a firm two-handed grip on you; a one-handed or just a solid connection where arms touch will naturally limit your control angles.