Strange under extrusion midway

[AtlasMaridan]

Dear all

We are experiencing some strange underekstrusion behaviour that occurs mostly when printing multiple objects. The pictured PLA print below is and example of this. both object where printed simultanious but when it got to the height where it was finished with the first print the quality of the second print suddenly went bad. There is clear signs of underextrusions and the outer wall is not connected to the internals. Then at the top it started printing just fine again. Does any have a good idea what this is, is it some kind of temporary blocking of the extruder?

We would really like to turn up the amount of prints from our Ultimaker 3 ext but so far we have not been able to print more than one object at a time due to various under extrusion problems when printing more than one at a time.

Looking forward to hear from you guys!

Kind regards

Martin