Top layer and walls have gaps even with 120 % flow (Cura) I'm printing with Geeetech A20 printer, PLA.
I've started printing the same test piece while changing the following parameters:
Temperature from 215 °C to 195 °C (lowering by 5 °C on each print)
Flow multiplier from 100 % to 120 % (increasing 5 % on each print)
Speed on two first prints was 60 mm/s and on two last 40 mm/s
Bed leveling is perfect, the hardware seems to be in tact, the nozzle is clean and still the walls on the outer curve print well all the time, but in the inner circle they seem to be under extruded (even though flow is up to 120 %)
I am planning on going on with these tests until I get the perfect print, but I am hoping somebody can guide me on what to try next.
I've been reading about the issues on so many articles that now my head is a mess and I just need some clear instructions.
You are lowering temperature and increase the flow at the same time - which is contradictory (filament is less runny, but you pump more of it). Though 195 °C seems to be very safe for printing PLA at 40 mm/s. Just doublecheck that when you disable motors, and push filament with hand, it melts quickly and easily goes out. Otherwise you will deal with temperature-related issue.
Outer walls are usually printed slower, and even when underextruded, part of line may appear constant thanks to leaking filament (it has time). Next retraction will cause stop leaking, and the other wall is printed with almost no pressure. Maybe your constant issue with &quot;inner circle&quot; is just a consequence of such moves.
My suggestions:

Did you calibrate the extruder (steps per mm)? - if not yet, do it (it's a basic skill); to quickly check if this could help, try to continue increasing flow by 5% and observe what will happen then, because maybe this is simple underextrusion due to incorrect steps/mm?
Are you sure that extruder motor is not loosing steps? (e.g. too weak motor against the pressure)
Are you sure that filament is not slipping? (e.g. blunt driver's teeth, or too loose idler)
Could you unintentionally enable volumetric extrusion?