Common problem with cheap Chinese high power optics....might say 4-16 or 6-24 but once you get to about the middle of the power range they become unusable due to clarity. It's just a poor quality scope, you gets what you pay for in glass.
Any cheap glass won't be able to handle a 4x magnification range ie 3-12 4-16 5-20. The greater the power range the more care and precision and need for high quality components becomes very critical.
If you really need to buy cheap glass, stick to 3x range like 3-9 4-12.....anything higher like 6-18 will just be unusable due to #### quality.