what can be the problem? it a PLA, 0.50 nozzle, Airwolf3D printer, 0.2 layer size. it’s kind of starting well, then starts to make non-solid lines…
Partly clogged nozzle?
PLA means it could have heat creeping up the hot end. Any extruder may have trouble with slippage in the cold end.
Do you have a fan attached to your hot end? Do you have pictures of your hot end and cold end? What temperature? Has the filament had a chance to soak up humidity? Do you hear any noises like hissing or popping?
Those are some questions that the PLA users will need to know to point you in the right direction.
Looks like not enough grip on the pla causing under extrusion.
first layer thicknes? Just had a whole discussion about that here.
The plaid indicates part of the problem is at the knurled (bolt, pulley ) drive. Could be a build up of PLA in the groves. Also could be the knurled ridges have been ground off by pinch bearing misalignment.
O found PLA in the groves. When just looking ar the speed of extrusion, I noticed inconsistent amount of filament coming out. Then I checket the grooved bolt and it was full of PLA I cleaned it now and i extrudes with consistent speed, let’s see how it prints… 
thank you for suggestions, I really appreciate that!
@Zviad_Sulaberidze Congrats on finding the PLA in your grooved bolt. Please be advised that it may have built up there due to another problem like a partial blockage, heat creep, trying to push the filament too fast or the temperature being too low. I wish you luck.
the print went well! thank you again!
Looks like heat creep, do you have a fan on your hot end barrel?
yes I have a fan
Just to make sure, you have a fan on the barrel and a cooling fan right (2 total)
I agree with @NathanielStenzel . Check all those possibilities. In my case trying to push the filament with the temp too low and caused the slow buildup of PLA in the grooves. This mainly happened during big fill areas when the head is moving faster.
For some colors I have had to raise the temp +15C over lowest temp color.
Sometimes, it’s just the nozzle being too close to the bed that starts the problem. Then when filament gets chewed up, it causes secondary feeding problems.
it was a problem with the filament in the grooves. I cleaned it and it’s ok now
@Zviad_Sulaberidze What a few of us have been trying to say is that you might have a series of problems with one causing the other and that the filament stuck in the grooves may be a 2nd or 3rd problem. Moisture in the filament or the nozzle being too low or the feed rate being too high or you extruder speed being too fast may cause the filament to receive resistance, so the extruder ends up grinding on it instead of successfully pushing it.
The grinding of the filament is in my opinion worse than skipped steps for the stepper motor, since it will eventually lead to the filament being chewed in half and not usable. If it was skipped steps, the filament would be fine and it would advance when the resistance lessened.