There's this idea about evocations that if you do them and you don't see the spirit, you aren't doing them right. Some of this attitude comes from Joe Lisiewski's books, with his focus on doing traditional evocations. The consequent result is this elitist belief that people who don't see a visible entity clearly are doing it wrong.
As someone who's done a lot of evocations, I can safely say that the criteria that you need to have a visible appearance of an entity to prove you've done it right is just a bit biased toward a traditionalist perspective of magic. Then again, I've also successfully evoked three people into my life, so that not only have I visibly seen them, but also physically felt them. To be clear though, I've never approached evocation as a mental experience that just happens in my head. The concept that an entity is just a psychological aspect of yourself is the excuse of people unwilling to apply themselves.
But what I disagree with is this notion that you have to do evocation a particular way in order to establish contact with the spirit of your choice and achieve the result you desire. Although I have a background in ceremonial magic, I've long since stopped using the props and focused on personalizing how I do magic. I figure as long as I understand the underlying principles, the entity in question isn't going care how I evoke it.
More importantly, if I'm doing it right, it won't matter if it visibly shows itself to me. I will know its presence. I will feel its presence. And maybe this just a case of semantics, but I've never visibly seen an entity, at least not with my eyes. I've visualized an entity and I have felt its presence in the room with me, but I've never physically seen one. Nonetheless I have successfully evoked entities and achieved measurable results with the aid of those entities. I've developed and maintained relationships with those entities. They have moved me.
It could be argued that if I took the time to evoke the entities using the traditional grimoires I might actually see them. Yet nonetheless, I know that my approach to evocation seems to work well. It's not traditional, because I find traditional boring. Nonetheless there is still real work involved in the practices that I'm doing. Genuine communion with an entity is not something that can be judged solely on if it appeared before you. Genuine communion is an experience and how that experience manifests may differ from person to person. Nonetheless if a person is genuinely moved by the experience, what speaks to the success is not just the manifestation of a result, but a true relationship with the entity that brings about a consequent change in the person's life that is truly sustainable.