Long prints shifting with Ultimaker 2 I am currently using Ultimaker 2+ to print some big parts with PLA. As long as the printing lasts for less than 15 hours (that's what I observed by experience, approximately), there are no problems. 
If the duration of printing is greater than this time, the part would get shifted from its initial position. It looks like the part is not sticking to the bed anymore. 

I was first using classic glue sticks and this problem occurred. Next, I tried with different masking tapes but the problem remains the same. The glass bed is not heated. The first layer and the following are actually pretty clean. I use brim. I reduced the printing speed to 20 mm/s on purpose to see if it would reduce the probability of this phenomenon to occur. However, this is not the case.

Do you have any experience concerning this problem, ideas, suggestions?
If the brim is firmly connected to the part and sufficiently large, you could "duck tape" the brim to the bed after a few hours; by then you should have enough space/clearance to work with. 

But maybe the best advice is to invest in a heated bed platform, or use the heated bed if you only did not enable it. Heating the filament to the point close to the glass transition temperature ensures that the printed filament stays ductile. Once filament cools down, it shrinks and cannot grip on the to build plate indefinitely.

My Ultimaker 3 uses a heated build platform that can virtually make every filament stick to it as long as you prepare the bed correctly. On glass I prefer to use a specifically designed printing spray called 3DLAC; that, and a little heat (depending on the filament you print; i.e. the glass transition temperature) make that I never have had any problems with getting filament to stick to the build plate. I'm even able to get the slippery filament POM to stick to the heated bed.