I have just bought an EleksDraw from Banggood. Having assembled it, I realised that the pen carriage is only held by 2 screws into the brass runners through the end of the Y carriage. This allows the pen to move if either of the 2 screws become loose Thus, the axes will not repeat accurately. This meant I've had to modify the carriage, as others have done, or check the pen tightness before every plot. My immediate solution, has been to drill 2 more holes on the end of the Y carriage, in line with the other ends of the brass slides and fix with 2 more screws. The other thing I noticed and found a useful help over was: the sticking of the carriage bearings, if you over tighten the 6 screws on the top of the carriage plate. If you push the carriage up and down by hand, when the motors are powered off, you should feel any 'sticking' or resistance along the movement. Obviously, such resistance, in places, will cause missed steps. After these 'mechanical' checks, you could then look at electrics and control.
Another accuracy test to try. See if you can get a drawing of a matrix grid, across the maximum drawing area and look for distortions to the squares. Then, concentric circles, that will use angular motion across 2 axes. If these tests work well, at least once, or a few times but degrade over time, it strongly suggests mechanical 'movement'. Hope this helps.