Obsidians cannot pushed by piston

Obsidians cannot pushed by piston. Pushing obsidians are less than 13, but they are not pushed. 

Steps of Reproduce

1. Place obsidians in straight line less than 13
2. Place a piston or a sticky piston next to the end of the obsidians
3. Send redstone signal to the piston

Expected Results

Obsidians are pushed by piston.

Observed Results

Obsidians are not pushed by piston.