Agree w all previous suggestions with a few more (some of these may be more or less applicable since you have success with other clients, my last point is my hunch for that specific client):
-All brands do have slightly different formulas and I've historically been pretty skeptical that that realllllly could make a difference functionally, however I've worked w chemists quite a bit (due to a personal gel allergy), and learned that it can/does. For instance, most "soak-off" gels are intentionally slightly undercured in their protocols to aid in easier removals, some more than others (Shellac vs Gelish for instance); also, curing times are important to follow, and Gelish specifically has a 5 second base cure (or slightly longer depending on your lamp). Over-curing believe it or not can be a problem...it's a bit odd to conceptualize, but that may be an issue.
-Which brings us to lamps. This may be controversial and isn't what manufacturers claim, but over 5 years of only using LED, incl branded professional lamps (cnd, gelish, and biosculpture gel) as well as back-up lamps from several manufacturers and cheapo ones, I firmly believe that even though the lights themselves look all functional, we really noticed they seemed to wear out a bit, needing a bit longer to cure than previously. We noticed this around the 1-yr-ish point with heavy use. I still don't know if it's in our head, but it happened with 3 of our professional lamps, which were all then replaced and seemed to fix the softness we'd experienced.
-Finally, and I swear I'd never even heard of this, but now I'm so confident of this as a possibility I ask the client almost immediately, and 9 times out of ten, clients w lifting issues answer with a "yes:"
Do they have any thyroid conditions, are in the midst of any major hormonal shifts (entering or ending menstruation, pregnant/nursing, or on hormone therapies), and/or have any auto-immune or cancer conditions or medications?
We have an incredibly loyal clientele that we've been able to service before, during, and sometimes after some of those conditions, and it really does seem to impact adhesion/longevity with gels specifically. I have zero explanation, but this is my hunch. For clients who are willing to experiment we found that different brands and combos of base/vs color helped ea of them differently, and every once in awhile nothing ended up working.
I hope some of these help! Xxo