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?