I noticed a glitch on the physics engine concerning the low end computers playing in browser.
If the rendering is chocking to drop the framerates enough the physics calculations go awry making it hard or impossible to get enough thrust to lift the piecess off the ground.

The fix is underway and will be posted in the next iteration of the game , earliest being next week since I'm preoccupied on the game jam at the moment on our next game project.

-Rami