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 https://academic.oup.com/jinsectscience/article/12/1/14/878432
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.