collider problem - objects go through objects

[mic3mic4]

I have a collide operator between a cloth (cuda) object and a ground. when I put a force (gravity) to it, some vertices of the cloth go through the ground when they shouldn't. 
The tyflow time set is set to 1/8 frame. the bounce interaction to 0. 
I don't know why that is or how to solve this.
Any suggestions? 
Thank you