Sticky pistons dont stick to other sticky pistons

Put the summary of the bug you're having here

What I expected to happen was...:
When i put a sticky piston on top of another sticky piston i expected it to stick to that sticky piston

What actually happened was...:
It did stick to the other sticky piston but it did fail to stick after it had contracted a few times

Steps to Reproduce:
1. Place a sticky piston facing upwards
2. place another sticky piston on top of that
3. wire it up with redstone
4. Then the sticky piston dosent 'stick'