If your 3D printer can't print circles. Here are a few reasons and things you can try:
Most 3D printers can print circles or curved shapes as long as the object is reasonable to manufacture, the material is good and the printer is correctly set up.
Top Reasons:
- Loose belts tension on extruders
- Unrealistic hole dimension for printers to print well (Too small)
- Poor material filament
- Poor bed adhesion
- Stringing
- Printing too fast
- Wrong nozzle size for the object you want to print
- Slicer software
- G Code error
Top Fixes
Try A Different Slicer Software

Before blaming the 3D printer or the material, actually, take a look at the G code and the slicer software first.

Sometimes G code conversion data is in the buffer, but the driver executes the file faster, which causes stuttering. In another word, not synched and the printer can't print smoothly.
Some slicer software creates more lines of commands of the same object than others.
Reduce Mesh Resolution

Printing unrealistic circular shapes with the wrong nozzle type or dimension can throw off the print. Be sure to understand the micron resolution before printing and set the dimension of the 3D model printable.
Reduce mesh resolution relative to the nozzle type and material first, then test the print to improve the overall quality.
Lower Print Speed

Printing the part slow is a common troubleshooting technique to reduce oozing and stringing to ensure each layer is built properly on top of one another.
Try a lower speed setting and test to see the result.
Upgrade Your Printer
Sometimes the best solution is to upgrade your setup to get better results. The print quality is only as good as the 3D printer and the material you use.
If you're using a beginner level 3D printer to print complex objects, and you're not happy with the result.
Please consider upgrading to a higher end 3D printer with better capabilities to eliminate some of these problems.
Click here to see what pros are using