It’s probably because Felblade activates the GCD and its getting stuck trying to run Felblade over and over again. Move it down to the sequence area and also add it to the Key Release so it gets a second chance if something else it tries in the sequence is on cooldown.
Phil-f is exactly right, you need to remove the line i bolded above. The addon is just going to try and cast only that spell and nothing else. If you want that spell to be used more often you can change the priority in your sequence section or you can add it to the other sections like key release, pre macro or if you have a loop limit set post macro.
Below I made a quick Macro that you were looking for and changed a few things such as modifier that I dont think you needed such as channeling (what are you supposed to be channeling this whole time) and some combat tags because you wont use them anyways unless you have a target. I left some in that I believe the DH would use without a target or combat though. Also mind you I dont play a DH really so there might be some discrepancies. Let me know if you have anything else.