Why Does Magick Fail?
If magick can be so powerful, especially for someone who has a strong relationship with the source of their magick - in our case the Santa Muerte - then why does it fail? In all honesty, I don't know, but everything I know about the world tells me that there's only one true certainty in this life, and magick isn't it, so there you go.
If I had to guess, I'd say that magick fails when: (a) we ask for things we don't actually think we can get, thereby defeating our own intent and reducing our rituals to motions instead of magick, (b) we ask for things we don't actually want because of how our subconscious might feel if that thing actually came into existence, again reducing ritual to motions, or (c) we ask for things we can't have for reasons unknown to us; the "grander plan" argument. But these are points made for the sake of discussion and, as already mentioned, I don't know why magick fails.
Plan Ahead for Success
Before we get into how to respond to failed magick, I should address how good planning can greatly improve your chance of success in the first place. Try not to rush petitions, and don't ignore the Santa Muerte until just those times when you need something.
Imagine if someone you thought of as a friend only came to you when they needed something and then rushed through the request as if it was your duty to help them - would you want to keep helping that person? It takes little effort to make a nice gesture that can go a long way toward getting the Santa Muerte in your corner. Instead of waiting until you desperately need something later, try lighting a stick of incense on your altar today, or burning a candle today, or reciting the rosary today, without asking for anything in return.
The gestures at your altar don't have to be tied to any request - it's okay to just stop in and say hello.
The Rule of Three
I personally follow a basic rule of three, which is, if I've asked for something three times and haven't received it, then it's time to take a hint. But there's more to it than that.
After my first failed magickal attempt, I'll conduct an internal survey to see how I feel about my goal. I'll meditate on the thing that I want and see if it's actually what I want, or if what I really want is something else. Do I really want my boss to drop dead? Or do I actually just want my efforts at work to be recognized? Do I really want to win the lottery? Or do I want to be able to get a kid through college? Do I really want to marry the person from work? Or do I want to marry someone like the person from work?
In the event an internal survey reveals that, in fact, I do still want what I was originally after, then I'll proceed to make a second petition, only this time putting more intent into it.
What do I mean by that?
The tools and correspondences we use in magick all help to concentrate and focus our intent and mental / spiritual energy to the task of shaping the world around us - in other words, they make our petitions louder and brighter. Most spells work with a minimum of magickally corresponding ingredients, but if your magick fails, it may be a good idea to increase the amount of magickal symbolism in your work. In other words, if a general use candle didn't get you what you wanted, then try a goal-specific candle. If a goal specific candle didn't get you what you wanted, then try etching something more specific into the wax. Of course, I'm not just talking about candles, I'm talking about your magickal petition as a whole. Every bit of magickal symbolism you can add to a spell or petition, the better.
Third Times a Charm (or Not)
If a second attempt fails to produce your desired results, there's one last option, at least according to me and how I choose to work with the Santa Muerte. If regular spells, prayers, and offerings have failed to get you what you want, then you can make a contract with Santa Muerte, which is a promise to do something in the future in exchange for a miracle or petition granted in the present. It may sound straightforward, but you have to make it something big, something you can actually deliver on, and something you can put your heart into.
Examples include a promise to recite the Santa Muerte rosary every day for the next 1000 days, a promise to erect and maintain a public Santa Muerte altar, and even a promise to give a piece of your skin to Santa Muerte's veneration. As you might guess, the bargain is a last resort, and should only be made when you want something so bad that you're willing to commit to a serious promise.
Your side of the bargain comes due when Santa Muerte grants your petition.
If this still doesn't work, this is where I take a hint and drop the matter, at least for now. If enough circumstances change in the future, I may try again, but they would have to be significant changes.