Pig is Hai Water. The Yang Jia Wood is supported and well looked after by the water. This is like the a guy who is interested in a girl, and the guys family supports the guys interest in pursuiing this particular girl.
The Snake, is Bing Fire. The Yang Metal, has every intention to combine with the Yin Wood in the Stem, cannot for whatever the reason Bing Fire wants. This is like Romeo and Juliet In short, Bing Fire is the Family Fued (doesn't matter which side really; but assume male side). The Yang Metal is Romeo, and Yin Wood is Juliet. But neither can go against the say of the most important (and strongest) influence: Bing Fire, which does not let Geng Yang Metal do anything it doesn't want. Therefore, it cannot combine like the rest despite it's desire to do so.
All together:
Ji (Yin Earth) over Hai (Pig Water): Ji Earth with Jia Wood. Wood is strong as it is supported by the main element, not an issue.
Ding (Yin Fire) over Hai (Pig Water): Ding Fire with Ren Water. Water is strong as it is the main element itself, not an issue.
Yi (Yin Wood) over Si (Snake Fire): Yi Wood wants to combine with Geng Metal. Metal is weak and subdued within by Fire, unable to combine.
Xin (Yin Metal) over Si (Snake Fire): Xin Metal wants to combine with Bing Fire. Fire is strong as it is the main element itself, not an issue.
Gui (Yin Water) over Si (Snake Fire): Gui Water wants to combine with Wu Earth. Earth is strong as it is supported by the main element, not an issue.
In short, it still combines (because it has every itention to do so). But also doesn't (because it can't get past Bing Fire who's in the way).