Some compatibilists—e.g., Vihvelin and Fara—think that something that merely blocks the possibility of your trying to do A but doesn't block your disposition to do A when trying to do A does not take away your present power to do A. Two examples of such blocks are (a) Frankfurt cases where you'd be counterfactually prevented from doing A and (b) being determined not to do A.
But there is an interesting family of cases where you can only do something when you try hard enough. For instance, you can run distance D in time T when you try really hard, and you can only try that hard when you know a bear is chasing you. In a case like that, even though you are disposed to do A when you try hard enough, anything that blocks you from the possibility of trying hard enough also blocks you from being able to do A. Thus the absence of a bear, or even just ignorance of the presence of the bear, blocks you from being able to running D in T.
So where trying hard to do A is needed for you to do A, anything that blocks your possibility of trying hard blocks your ability to do A.
Now, anything that blocks you from the possibility of trying also blocks you from trying hard. So in cases where trying hard to do A is needed to do A, determinism and Frankfurt cases block you from being able to do A.
So in cases where success requires trying hard, blocks to trying remove the ability to succeed. But why should this only be true where success requires trying hard? So in cases where success requires trying, blocks to trying remove the ability to succeed, too.