Hmm, maybe - maybe it’s that the situation is perfect for the higher strings, and the hand and the motion is ideal and primed for efficiency when executing notes there, but then when you move to the other 3 strings, maybe there’s some kind of obstacle - the hand is crashing into something, the angle changed so efficiency drops, or maybe it’s not that efficient in the first place. I don’t know - I can only offer an opinion based on my own experience.
I worked like the devil trying to get something that was marginally successful to be extremely successful, and like you I had a spot where maybe 10-20% of the time I’d experience success, BUUUT because it was the closest thing to success I continued to beat it to death and very disappointing results for the time and energy expended.
I would say that if you are getting garage spikes 80% of the time, that is a problem - and no amount of “working through it” will get you the results you seek. Something has to change I think. I think the experts here could tell you a lot if you posted some video!
The advice I received from Troy and Tommo was a lot of “effortless, efficient, and smooth” in regards to the motion. Without that, it’s dead in the water. I couldn’t see the forest for the trees at the time, but having spent the last couple of months thinking about those statements - I really believe that’s the most important first step. For me, anyways. lol I always come up with these godawfully nightmarish things that I want to be able to pick, and I tend to try and work on those - but really it’s getting that aforementioned smooth feeling with repeating notes on all of the strings. I have realized that if I can’t do that, then I need to stop investing time into something that doesn’t work and do something else. That’s just me, though.
Step 1 - Easily be able to do a fast tremolo on all strings.