I dont know if I understand you correctly, but let me try...
It could be that the wall-thickness of the part you try to print is to thin or is not "compatible" with your nozzle size.
Lets say your nozzle and output size is 0.4mm. You now try to print a wall with a thickness of 1mm. So the printer will print 2 wall-perimeters (left and right) with a 0.4mm thickness. Between these walls there is now a 0.2mm gap. The slicer cannot fill this gap as its to small for the nozzle, so it is left empty.
I run into this problem multiple times already with different models using Cura.
But, atleast in older versions of Cura, if you go to the layer-view you can kind of preview/see the problem.
(Iam still using an older Version.)
This problem can also effect the overall print quality of the part.
(For example I had a part with very big stringing problems but only on the inside of a specific wall. The wall wasnt even thin, it was a couple of mm, but the overall thickness of the wall was not a multiple of my nozzle-size so it created problems.)
Some say you should design a model with a nozzle size already in mind, however this is not really practical, as everybody uses different settings/printers and not everybody creates his own models.
Here are some more infos regarding this issue: (and more "walls-not-touching" issues)
So maybe try varying your nozzle size and wall thicknesses and see if this changes something in the cura preview already.
This problem can also happen if you have some very tight corners, which are to small for the nozzle.