my guess would be a MAG deficiency... try adding some dolomite lime... Check the PH of the soil if you can... could be a nute lockout. My plant did that exact same thing and I added lime as a top dressing and pushed some water down.... Lime has calcium and Magnesium and will act as a PH buffer to not get a nute lockout... You still have to keep an eye on the soild PH though.. Hope this helps
Elway