Having Trouble Progressing In Speed/alternate Picking

Hi all, I have a question to make sure I've come up with the right solution and am not pissing my time away. My picking hand technique is good, and my fretting hand technique for the following patterns are good

12-13-15 and 11-13-15

for some reason though, I have trouble with 12-14-15, especially when alternating between that pattern and the above two patterns on the same string.

Sorry that this is kind of a stupid question and I have started practicing the 12-14-15 pattern using the pointer ring and pinky by itself and then alternating between that and the 12-13-15 pattern using pointer middle and pinky alternatively. Is this the right path? I'm just confused why I'm so weak here when my speed picking is otherwise pretty decent, I can play the intro to Scarified decently.

So should I continue as I am? Is there a more effective way to practice? Or am I missing something entirely?

EDIT: Just to clarify why I'm confused and frustrated, I thing Muris labels as level 5 without even having to practice any of the parts but I'm having trouble with this one solo he has marked as level 4 because it has the alternating patterns on the same strings.