Latte Art - Help?

I just got a Rancilio Silvia. I looked through the many articles concerning the Silvia, including those specifically concerning latte art. I got myself a stainless steel milk jug with shaped spout designed for latte art, and a thermometer and clip from a barista supply store.

I practiced the technique with skim milk (easier) and was more or less successful. I tried again with half-and-half (2%fat) and have not been successful. I have tried over 10 times.

I either get what I call a "foam" or a "fizzle". In "foam", the top foam comes out and already fills the top of the cup before the level is halfway. In "fizzle", the level goes up and up but the "white spot" that is supposed to signify the start of the latte art move, never materialises.

Additional information: I pour for 2 cups at a time, the cups are not the semi-hemispherical shape but not exactly a cylinder either.

I suspect that the milk has separated but I thought I did the texuring correctly.