It could simply be a faith issue and not in the theological sense. You may have a lack of faith in others spells which could affect your confidence during casting which is crucial. It would make sense to have more confidence in your own spells especially with the results you have spoken of. OR others' spells may contain certain falicies that you weren't aware of and thus resulted in a void spell.
It may also be that when you design a spell yourself, you have a deeper, more personal connection to the spell, your goal, and the process of connecting one to the other. The more you relate to the actions of the magic, and the more you relate those actions to your intent and the idea of setting it into motion, the more effective the magic will be.
When using a spell that was written by someone else, it is more challenging to feel that same connection because you don't have the same mindset or associations as the writer. So the steps you take don't have as much meaning for you. This creates a bit of a void between yourself and the magic you are trying to work, where your concentration and intent aren't entirely on the goal any more, but split with other things. Like wondering if it will be successful. Remembering what needs to be done. Wondering why a particular symbol or gesture is supposed to be used, and that sort of thing.
Spells are always more potent when you write them yourself because there is more of yourself put into the creation of the spell. More thought, more desire, more feeling, more power. Only you really know what you want.