Over a long period of time the teeth wear and biting pressures change. These forces apply undo pressures and some screws lose their grip. This is a normal process, this is why we always check the bite during the cleaning sessions, and provide adjustments if needed.
However if screws come loose within the first 3 months, there usually is a mismatch or a fit issue with the components of the implant complex.
Dr. Herman Kupeyan