[Bad printing quality after 5mm height]

Hi,

I have been using ultimaker 2 go for 7 months and its printing quality is good so far.

However, it messes up ( underextruding ) recently.

Here is a test of printing 3 ultimaker robots to show the problem and the result are as below:

 

as you can see the printing in the beginning is good and it messes up after 5mm height.

I had went through the Atomic method so I know that the nozzle is 100% clean.

Do anyone have the the same problem? How to fix it? Please help!

lam