Are you not rinsing the One Condition out properly as per the CG book? Leave in can be really useful for encouraging clumping and preventing frizz, you can use OC you don't need another product. Many curlies need more conditioner if they choose to shampoo. Are you doing deep treatments?
The other obvious option is try a different conditioner wash product, don't quit on the first one. Deva can build up on some people but if it's only in one spot TBH you might look at your wash and rinse technique, you might just not have been cleansing that area properly. You can also alternate shampoo and co-wash it doesn't have to be all or nothing, or do condition-wash-condition.
Are you scrunching the water out before or after you apply all your products? See the techniques sticky on this board for various product application and clumping links.
2a-2c, medium texture, porous/ colour treated. Three years CG, growing out mechanical and chemical damage = breakage and very high porosity. Past armpit length heading for waist.
CO-wash: Inecto coconut
Treatments: Komaza Matani, coconut oil, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss ultimate volume, Inecto argan
Styler: Umberto Giannini jelly, Boots Essentials gel
Experimenting with: going back to basics