Android's ListView randomly without separator

Question

I'm removing the divider png's from my app, and replacing it with a simple black line.
I thought that android put a simple default divider (1px black) between items, but I have it only in some ListViews... why?
If I try to insert an android:divider="@android:drawable/divider_horizontal_dark" and/or android:dividerHeight="1dp", I don't see any divider. Again, why?

Answer 1

Use 1px as dividerHeight, and not 1dp. 1dp will mean 0.75px in 120dpi devices - which is rounded off to 0px.