Sprite falls partially through ground

I am testing the following:

brown sprite: player (platform object).

green line: ground (solid object).

<img src="http://s16.postimage.org/xzkwxt4o5/scrshot.png" border="0" />

The brown sprite falls through the green line in the area right under it, but can stand on the line in the middle part of the green line.

I wonder why. Is it too thin?

Have you checked the collision polygons?

Thanks, it's solved!

I didn't check the collision polygons.