I have planted around 5000 of them and faced the same issue (even now the plants do not have uniform growth). Similar to you I had provided same amount of fertilizer from the beginning.
Later on I provided more urea to the underdeveloped plants and I saw that the growth rate increase.
Another important thing is to provide adequate waterand keep the plot ‘cool’. In couple of my plots (which did not have shade) I noticed that due to scotching sun and due to delay in watering (I do flood irrigation) , the leaves had turned yellow and growth had stagnated for some days. After providing water couple of times the new leaves that were emerged were all normal and green.
Good luck.
-Biju