My kossel prints fine. The only problem is that first layer (and first only) is uneven: somewhere is smooth, somewhere lines are visible (check photo!). What’s the problem?
May want to adjust your extrusion multiplier up ever so slightly.
Using a piece of paper, check if the nozzle height is the same across the print bed. Imperfections in the carriage or rods can lead to the nozzle shifting up and down slightly when moving sideways, that can lead to an uneven first layer. Welcome to the world of pain that is delta printers 
@Shachar_Weis I’ve tried after a g29 and it seems fine…
@Andrea_Fontana bed probing won’t help if you have structural imperfections. Do the paper test.
I’ll do another try tomorrow with paper then. Too late here
Under extruding maybe. the first action is to manually calibrate the print plane as suggested with paper, by changing M666 values for XYZ.
Then the print plane will be convex or concave in the center. Next change delta radius in firmware, if center is too high then increase Delta radius or decrease if too low.
Look on youtube for Jay Couture’s video on bed leveling.
I used Openscad to create a Triangle so the three corners would be right in front of each tower or I can rotate it so the corners are opposite each tower.
if opposite the towers the point is too high then increase diagonal rod length or decrease if too low.
Measure and change M666 values to get the first layer to “squish” properly so there is no gap in the first layer.
What slicer are you using? Looks like a need for pull back on first layer travel speed with the current layer height.
the head is not running parallel to the bed or the otherway around. or maybe both.
I tried to tune with m666. That gives an improvement on quality but still the problem on first layer stay there
(Just on one half of base. The other is fine)
If only half the base, then for that half the nozzle is too high. The print plane might be tilted up on one side and down on the other. To get the M666 values closer print a one layer triangle or square you can design in OpenSCAD or use the object you are currently printing, just stop the print part way through the first layer and measure the thickness of the first layer making sure you match the measurement with the closest tower, then adjust M666 value accordingly. Too thick, then increase M666 if too much squish then reduce M666 value.
Delta or cartesian? Is the half of the first layer that prints funny the first half of the layer to print or the second half or some mix of the two?
I agree with the idea of a slanted bed or that the nozzle is not touching the bed all of the times that it should.
I have two nozzles on my printer now and the effector on this delta had some wobble until last week. I think I have fixed that, but I still need to confirm that. It was causing my prints to be sliced by the 2nd nozzle, but it could just as easily cause some of the layers to not squish enough. In fact, it was doing both.
I had a similar problem that my first layer was always uneven. In the front it was creating a perfect adhesion of filament to the printbed, and on the back it was a bit to close, leading to too thin extrusion there. In my case I had to adjust the tower position by carefully changing the value of DELTA_TOWER1_Y to fix it.