Issues assigning hardware faders on Mainstage

Hi there. I have recently learnt to assign my hardware faders to the virtual Mainstage faders, by starting at the concert level and then overriding the concert level assignments at the patch level. Not sure if it's supposed to be done that way but it works!

By patch I mean that I've linked 3 or 4 different patches to create a new combined sound, and am assigning a fader to each of the component patches within that new sound.

However I find that, depending on the patch, sometimes I am able to assign all the faders. On another patch I can only assign two or three faders. Sometimes inexplicably the fourth fader won't be assigned (this is all at the patch level, not at the concert level). There seems to be no rhyme or reason: sometimes they assign, some they don't.

I'm not sure if it's a glitch with Mainstage, or with my laptop (it is very old: Macbook Pro from 2014 running Mojave). Everything else seems to work a treat though and this is such a weird problem.

Apple helpdesk can't help because the computer is so old. So to anyone out there, can you? Thank you