Second layer: gaps, poor adhesion - why? I'm seeing the following pattern on my Prusa clone:



The first layer prints OK, layer adhesion to the bed is good. The second layer shows the weird gaps. On larger pieces, or with an increased feed rate, it sometimes gets so bad that parts of the second layer detach and are dragged across the print. I get the impression that this effect is worse when printing in the "top left to bottom right" direction, but not as bad when printing in the "top right to bottom left" direction. 

This occurs with multiple filaments and materials. I've already tried to manipulate the flow rate, temperatures and print speed, but gotten little improvement if any. 

What might be causing this issue?
I wouldn't describe that as an "OK" first layer. The nozzle is either too far from the bed, or you're underextruding. Underextrusion also looks like the cause of your second layer problems.