i think we're all pretty much aware of the usual culprits for "real" thin ends -- breakage, dryness, overmanipulation, mechanical damage, over/underprocessing ... but
what if your hair is still "thinning" and you're doing everything right? which seems like the scenario the op is talking about.
i think we also need to remember that hair naturally grows in unevenly, which can look like "thin ends" -- but its actually healthy strands that have:
a faster growing rate, or are from follicles placed lower on your scalp ( ie usually the back/nape & sides)
most stylists (at least the ones who know how to correctly cut/transition hair as its growing out
) will keep the bottom layers even as the top layers grow down. so when the top meets the bottom layers you have a blunt, even style. growing forward, the stylist will then maintain this line so the hair "appears" to grow out blunt & even.
but essentially you have to cut blunt, evenness into your hair. hair naturally grows in staggered, "thin" layers. now whether you trim as you grow, or grow & then trim, is up to you, and your comfort level/style. some people hate to see "thin ends". some people don't mind it. there are benefits & detractions from both approaches.
but essentially, as long as your hair is
healthy, you don't have to worry that your ends are "thinning" because of damage, it's probably just from growth.