By tulsi plant, you (I think) mean Holy Basil, or Ocimum tenuiflorum. I'm wondering 1) how big the pots are, because its hard to tell from the photograph [which is upside down, by the way] and 2) how long the plants have been in the same pots.

If they've been in the same pots for a couple of months or more, and the pots are only six inch pots, then I'd say they've run out of root room and now need to be potted on into larger containers. I note also the pots are possibly sitting in what appears to be a trough to catch the water - if you don't empty out excess water 30 minutes after watering, the plants will suffer greatly, they do not like having wet roots for any length of time.

I don't know if any of this helps because basically, there's not enough information in your question for a helpful answer, but the points I've made might enable you to decide for yourself what you're doing wrong - if not, please supply the missing information, along with how old the plants are, that is, how long have you been growing them. And a picture the right way up would be useful - it probably looks the right way up on your device, but you need to turn it so its the right way up on the site.

If the soil that you are using as part of your mix has any clay content, then the soil structure will collapse, and cut off air and drainage to your plant's roots. Instead of soil, I would use sharp sand, or pumice or lava sand, if available, no more than 1/5 of the mixture.
Also, you don't mention any kind of feeding, and any Basil in a container is going to need a constant source of nitrogen. That in the vermicompost was probably exhausted after the first month. Bamboo's concerns about the size of the pots are valid even if you feed regularly--I would have each plant in it's own 30 cm pot.