Cura Sequential Printing
If the height of the object is larger than the free space between the gantry and the top of the first printed object, you will not be able to slice the object. The maximum printing height will reduce to the maximum available distance between the gantry rods and nozzle.
Note that the correct Gantry Height needs to be set in the printer properties. For instance, the UM3 I use has 60 mm free space between the gantry and the nozzle:

If you have a smaller height specified, or a taller print object os depicted in the image below, you will not be able to slice the prints (note that a 100 x100 x100 mm is scaled to 20 x20 x70 mm prints, which exceed the 60 mm):

When properly scaled below the 60 mm, i.e. to 20 x20 x50 mm print objects, you'll see that you are able to slice the objects:

Slice preview: