Over time and through repeated use, the hydraulic oil in your tools will degrade and become less effective. This is a normal process and happens for a few reasons.
The higher the pressure in a hydraulic system the more the oil will compress and degrade. But arguably, the most common reason is because of contamination. If you ignore the warning signs and do not replace the hydraulic oil the performance of your equipment may be affected, and the oil will lose its ability to defend against friction and corrosion.