Sanzoni,
It sounds like you have a happy little guy!
There are many reasons for plucking. Some of them are inter-connected. Not all reasons are understood or verified. Some people believe it is solely nutritional, others claim it is hormonal. First thing to do is have your vet run a complete blood panel to rule out any possible illness or dietary deficiency. Keep doing what you are doing with toys and enrichment. As CLB suggested, you may want to invest in full spectrum lighting or find time to take the bird outside in good weather.
Also, if you tend to focus on it a lot - change that behavior. Parrots love drama and attention. The more you fuss over plucking...the more you may get plucking.
If your bird has been plucking for a long time, it may be habit now - like biting your fingernails. It can function as a comfort behavior. In addition, if the bird is severely plucked, the follicle may be dead and feathers unable to grow back in. Do you have a photo of the bird?
I hope some of this is helpful! If the bird is healthy and happy and the plucking is not destructive, I wouldn't worry too much. Just get the vet to take a look. It's always good to have a yearly check up and the blood panel can indicate any problem areas.