There are two main (typical) ways you block manifestation. Learn what they are so you learn what to do about them.
The first block to manifestation is not asking for what you want at the start, or if you do ask, not asking in the correct way. Many spiritual disciplines provide versions of "Ask and you will receive," but there's more to just asking, as you've probably learned, and which I'll explain.
Okay, we get that we need to ask; but what usually happens is we jump into making a plan or take massive action, often because others expect us to or told us this is the way to act, or to appease the ego. We rush around, exhaust ourselves physically, mentally, and emotionally and feel frustrated about minimal results, or the absence of them. And, thinking something "to death" or worrying about it is NOT asking.
An old adage says, "Sharpen the ax before you go to the tree." Ask in the right way first. "If (insert your request) is in my highest good, I ask this to be fulfilled, and I trust that it will be. While I wait to receive results or guidance about this, I'm going to be happy or serene now, rather than wait to be happy or serene if or when I receive this or something even better. I am truly thankful for all that fulfills me." Or, "I ask that the right (clients, customers, resources, monies I need) be sent to me. While I wait for these to show up and trust that they will, or for guidance about this, I'm going to be happy or serene now, rather than wait to be happy or serene if or when I receive fulfillment of this request. I am truly thankful for all that fulfills me."
I know a number of action-driven people might disagree, at first blush, with the above paragraphs. We (and they) have been taught that strategies and systems and actions are what "responsible" people do, but we're seldom taught how vital it is that we strengthen and trust our relationships with our Infinite Selves and Source. Yet, these relationships are what are needed as our foundation, are where real inner power comes from and where fulfillments get created and put into motion in our lives. Too often, much too often, we attempt to do everything on our own, without the assistance of our Infinite Selves and Source. I'm not saying you aren't to take action, I'm saying get aligned with your Inner Energy before you take action, so that your action is Source and Life Energy-driven, not ego-driven.
This leads to the second biggest block, which is a companion of the first one: Strife. Notice that "happy or serene" were included in the requests shared above. Strife is a really big problem when it comes to manifestation because the vibration of fulfillment, the vibration that allows and supports fulfillment, is serenity. There's no way around this. Judging is strife. Anxiety is strife. Anger is strife. Stressing is strife. Anything not supportive or serene – anything not a positive vibration is strife.
We often call what we do "striving to succeed" or, perhaps even, to survive; but we're really "strifing," which will never result in the full success or fulfillment we desire. Strife blocks manifestation. If your ego wants to disagree with this, just consider your typical experiences and results. You can ask and ask (or plead or demand); but as long as strife instead of serenity is present, the energy of strife will push away the very results, resources, or people you wish in your life. By the way, serenity doesn't mean you're so mellow you can't function; it means your actions or non-actions, your words, your thoughts and feelings are centered and fueled via your connection with your Infinite Self and Source, instead of spiking out and poking people's eyes out… energetically speaking, that is.
The more inner strife you release the more outer strife will diminish from your life, and the easier it is for your Good and Fulfillments to reach you. This is the Catch-22 it's so easy for all of us to find ourselves engaged in. This may help: Think of a sailboat. Is sailing smoother, faster, more effortless on calmer waters or on stormy seas? Effortless manifestation is a form of sailing in life, and is worth doing what's required to know you can do this consistently; but, it is way more about first Being than Doing.
You cannot posture metaphysics or spirituality AND expect what you ask for to manifest fully if strife is driving your life. You don't even have to believe or practice metaphysics or spirituality for this to be the truth. Life responds in this way no matter what your belief system is or isn't. The more strife you let go of the more serene you'll be. Obvious, but we miss it more often than we see it. To do this requires dedication to develop your strife-free inner power, and daily self-discipline about this, so that you shift away from ego's insecurities that cause you to engage strife. If life doesn't seem to be working out for you, it's likely you're allowing strife to clutch you in its grasp instead of you holding serenity at your core.
Begin to practice looking at your blocked life areas to reveal any strife you may be engaging in. Ask for what you want, in the right way. It is your choice to either wear yourself out while you try to do everything yourself or to follow a path that leads to more effortless manifestation and greater serenity, fulfillment, and joy.
Practice makes progress.