I agree with blacktresses that HEAT 1-2 times a week may be the culprit, even the best thermal protection cannot totally avoid damage, merely reduce the damage, but over time, it adds up and the little pieces you see will prevent growth retention, therefore, harder to keep your length.
I've learned the hard way that my 4b relaxed hair cannot take heat, period. I tried every which way but loose to get around this and in the end, I totally eliminated heat, which was the HARDEST thing for me to do 'cause I love to wear my hair loose and bouncy, swinging in the wind and the fastest way to get there is a blowdryer, flat iron or rollerset under hood dryer. It's been 1 year since I gave up heat and I've retained 3 inches in that time, hitting brastrap my last touchup earlier this monthh. I continue to wear my hair out and swinging and bouncy everyday -- I rollerset and dry under hood dryer on *cool* setting and get same bounce and body as with heat -- only thing, it takes me 3 hours to dry on cool, rather than the 1 hour with heat. It's been a small sacrifice well worth every minute under the dryer. Also, because it takes 3 hours to dry, I only wash once a week 'cause no time to do more often than that and I found that washing less and eliminating heat = growth retention and fabulous body. Wow, loooong response that I hope helps . . .