Circle problems

I'm having problems doing proper round models but more complicated models come out great, heres a few pics :





The last image is the problem, but not very good photos im afraid, its a threaded lockscrew but because its more square than round its causing a problem. I dont understand why i can do a complex model without issue but have problems with anything that has to be round, any ideas ?

tia

I can't tell by the picture. But circle issues usually indicate either improperly calibrated steps for one or each of the axis or a backlash problem. Possible to get a better pic?