The problem is solved. I thought that interruptions were gonna be the answer to my problem but I've found another way to solve it.
I was using 'fors' inside the loop so even if data was available on the serial, I couldn't manage it until one of the fors was finished (and they were very long fors). Now, just calling the method inside the fors a lot of times (the same number of times than the fors counter), I can watch the serial constantly.
Thanks for your time!