This may be one of those problems with 3D printing that you can live with, although it can be an early warning sign of worsening under extrusion.
Signs of weak of under extruded infill:
Your 3D printing infill looks weak or under-extruded (spongy in appearance).
Explanation:
Most likely the layer isn’t missing, but due to a mechanical issue with your Z-axis, there was an uneven step when moving up to the next layer. Can easily be mistaken for [Under Extrusion] or [Cracking or Layer Separation].
How to fix:
Reduce the Z-axis movement speed in your slicer settings.
Make sure your Z-axis can travel freely. In some case, if the filament doesn’t unspool smoothly from the reel, Z-axis travel can be blocked or in a direct drive extruder setup the extruder motor might yank the extruder upwards.
Check your Z-axis, Clean and lubricate your smooth rods, rails, lead-screw etc. and look for broken bearings. If the error occurs exactly on the same layer in repeated prints, there could also be an issue with a bent or misaligned lead screw.
Pro Tip: check acceleration and jerk settings for the Z-axis.
Check the exact resolution of your Z-axis (motor steps and properties of the lead-screw) and set layer heights to use full steps. Sometimes, a 0.12 layer prints nicer than a 0.1mm.
If this is a newly assembled DIY printer kit, look into the micro-stepping behaviour of your stepper drivers. If this is a pre-built machine, contact the manufacturer for hardware support.