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. Past bra strap length heading for waist.

CO-wash: Inecto coconut/ Elvive Volume Collagen
Treatments: Komaza Care Matani, coconut/ sweet almond/ fractionated coconut oils, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss Ultimate Volume, various Elvive
Styler: Umberto Giannini jelly, Au Naturale styling gelee
Flour sack towel, pixie diffuse or air dry.
Experimenting with: benign neglect