I don’t see what’s so complicated. A circle is useful for evocation just to designate what space is yours and what space is the spirits…and it can help to mentally put you in the headspace of performing an evocation. But its not a requirement to summon a spirit. The problem is that you really are too new to all this to really know that…and from what I’ve read here you seem to treat magick like a chemist or engineer. Magick is not an exact science where if you add X and Y together you get Z 100% of the time. Just because spirits don’t always succeed with a particular request…by no means does this mean that they aren’t powerful. That would be like saying because Tiger Woods missed a hole, he’s not an excellent golfer. They usually do pull it off, as long as you’ve done things right…and that is what matters.
I would suggest, since you seem to have a lot of doubts about the ability of demons/spirits to bring excellent results…that you read this thread I made some time ago, cataloging the success stories of users on here over there years, so you can get a better idea of what spirits and magick in general is capable of