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:
Often due to settings being slightly out, while you may get away with poor infill not affecting the external appearance of your 3D print quality, it provides almost no stability to the print.
How to fix:
Reduce extrusion settings
Most slicers automatically increase the infill 3D printing speed or use a bigger extrusion multiplier/line width than regular outlines. This setting is often proportionally set by applying a multiplier to your general 3D printer settings. If you’re pushing your printer to its limits in speed and extrusion volume, the infill is the most likely place where it starts to fail first. Therefore, either reduce your general extrusion settings, or find the modifier for the infill and reduce that.
Increase heat on faster prints
You may be printing too cold for the desired print speed. Printing faster or at higher extrusion rates requires more heat to melt the proper amount of plastic in time. Increase your temperature in 5°C increment tests until the issue is resolved.
Lower layer height
You’ll experience infill issues if you’re trying to use a layer height that is too high for your nozzle or extruder. Stick to a maximum layer height of 75% of your nozzle size (i.e. 0.3mm for a 0.4mm nozzle).