Nozzle doesn't extrude after switching from 0,4 to 0,25

Hi everyone!!

I’m writing this post because I found some problems after switching the nozzle diameter from 0.4 to 0.25.
I use the machine at work from one year or more and we always had no problems with 0.4 nozzle.
I know I have to change some settings on the slicer cura… and I tried to do that but when the machine has to start no material comes out to the nozzle.
I also read someting about the extruding calibration but if no material comes out it is pretty difficult to try it :')
I don’t know if it is related to the extruding speed o else… however I tried to start the piece several times and everytime I have to demount the nozzle and clean it from the unmelted plastic.

I’m sorry if it is a stupid question but I never had the need to change it so I have no experience if not the classical print…

Many thanks