Sorry to hear that the GoM practices did not work for you. I found them quite useful, with a lot of results and many others did too (at least on Facebook groups). But like what many others have said, you might not resonate with the approach offered by GoM and this is totally fine. Sometimes it is the method that does not work for us and other times it is what we think about the method. For example, if you think the method sounds "too good to be true", than this can create resistance in the working and prevent you from actually getting the result. Such things can happen in magick.
I have also found results outside of GoM, such as with doing Solomonic evocations or using certain mantras. I think this is what is wonderful about the occult, you get to explore different things and find out the practices that work for you. When I started doing some of the internal alchemy work from Franz Bardon's work, I actually found that these worked far better for me for inner work than just using GoM magick by itself. I actually started sensing the inner changes that occurred within me in better time scales than anything I did using their works. So you need to experiment and try the different things that work for you.
One thing that we do need to make clear though is that while GoM (and similar Power of Magick works) do dilute the complexity of the magick, their magick is largely based on workings that have existed in older manuscripts and works. For example, a lot of the stuff they have in "Demons of Magick" is either based on the method of Dr. Rudd (think "Goetia of Dr. Rudd") and some practical Kabbalah. Another example is the "Angels of Alchemy", which has a lot of its method based on how to work with the 42 letter name of god and its offshoot angels. The 42 letter name of god and its angels are explored by experts like Jacobus Swart who proposes a similar, but more complex set of workings. Just because something is more complex does not mean that it is more effective. In all the cases that I mentioned, GoM works use material from previous works that are legitimately powerful rituals in their own right, but they ended up demoncratising it by making it simpler.
Others have mentioned that sometimes, the issue could also be a matter of fundamentals. Magical ritual requires powerful intention, focus, and direction to be effective. If your ability to focus and present your intention is weak, then your workings could fail regardless of the method you choose. If your ability to concentrate and direct the energies of the ritual are lacking, then similar issues could occur. There is nothing preventing you from working on meditating, energy work, or whatever in order to get more effective results. I use Initiation into Hermetics, spaygrics, and meditative practices to refine what I do when I work a GoM ritual. I find that it does help when you work on the fundamentals and channel your skills into the actual ritual. For example, in GoM it is very helpful to be able to feel the necessary emotions during the "emotional transmutation" process they usually use. If you meditate frequently and practice your visualization skills, then you might go further than feeling and actually visualize the emotions + the results you are looking for. The combination of emotion and visualization can lead to a stronger "intent" given to the ritual and could help make the ritual more effective. Small things like this can be overlooked inside of any ritual.
As a beginner / "neophyte", it might be easy to think that the magick is not working, but as you practice rituals more and iterate over your failures, you will understand more why certain things work. Others have said that you should record your results in order to have a log of results so that you can analyze what to do better next time; this is something that is extremely helpful in the long term. While certain people might advocate to understand one specific path or system at the start, I would advise that you explore far and wide in order to try and understand the nature of magick itself. Every magical tradition does magick and all of those magical practices end up having similarities to each other. If you understand their internal structure, their parallels, and their common fundamentals, you will come to understand the reality of what effective magick can look like. It also sounds like you are looking specifically into practical magick and so you might find it effective to read something like "The Elements of Spellcrafting" by Jason Miller. He gives a much better version of the concepts needed to do effective practical magick than the GoM does. I would suggest you read that and pair it with a practice that gels well with you which gives you a strong sense of the fundamentals.
