Troubles with quality in my UM2+

[pitmaster40 0]

Hi, I need help with a little trouble which is affecting the quality of my prints.

I've recently updated my Ultimaker 2 to Ultimaker 2+. But I could notice that something weird is happening without finding a solution. It seems, this printer is not closing properly the outer wall line by letting a tiny hole. As a result, you can notice these tiny holes randomly on the 3d object's surface.

This problem is more noticeable when you print at 0.25 mms.

What could it be happening with this problem ?

Thanks in advance for any advice you can give me in order to solve this.