I'm not certain what the problem is - the puckering and wrinkling of the leaves might indicate a viral infection, but I'm not seeing any yellow bars or spots that often go along with a virus, though that particular symptom isn't always produced. If it is viral, it will get much, much worse.
It's also possible that the new leaves were nibbled/damaged by something when they were very small, before they grew and opened out properly - that would also cause distortion in the growth. That would also explain why you can't find any insect problem currently - whatever caused the damage has either already gone, or does mnot actually live on the plants. In parts of India where castor is grown as a crop, it is usually not grown as a mono crop - there is a study here: Intercropping for management of insect pests of castor, Ricinus communis, in the semi—arid tropics of India, with evidence to demonstrate that growing certain other plants in amongst the crop helps to significantly reduce pest problems, what's known as intercropping. Next year, don't attempt to grow a mono crop of castor - it's worth interplanting in the way suggested in the link.