S’true! I remember having more air-prints and good ones until I changed the fake J-head that came with my first kit for a better hotend. There were times in the early days when it coulda been a bird or a plane… Right out the window.
This is what I just came home too…see? Happens to everyone. missing/deleted image from Google+
@Daniel_Jackman rotate it 90 degrees and there is less of a chance of this happening. Especially if you are trying to print 3 at the same time. 
I always meet such problem before, most case is because the base get loosing. I try many ways, yet not found reliable method. When print time increase, the loosing possibility get higher.
You may try use blue adhesive tape, when the base be done, pause a while, glue the base on the tape, then continue…troublesome, but better than face nightmare of printing another 24 hours or more…
I have another idea, every product use the same plastic base, after the base be done, then use square metal block fixed it, continue print…maybe need serial square metal blcok because product size are different. Yet not have time to test the idea, if someday prove it, will let you know.
@Neo_Qin the issue you’re describing is bed adhesion, my issue was a temporary jam in the hotend. If you’re having bed adhesion issues try raising the temperature of your heated bed.