Definetely a nutrient deficiency, hard to tell which one from your pics. I would suggest a fertilizer that contains all the required micro and macronutrients. Your garden centre should be able to help you pick an appropriate one.
Some nutrients (Boron, Sulphur, Calcium, etc.) Are what are called "non-mobile' and when the plant runs out, the new growth shows deficiency. Other nutrients (Iron and Nitrogen are the most common) are "mobile" and when the plant runs out, it steals from the old growth to feed the new growth. I would guess this is your problem but some better pics/a detailed description would help immensly.