This can be normal under heavy loads and extended use (especially for the Tilt axis) but can be corrected by the user. Please have a look in our NT Head user manual in our support section where you find instructions on how to recalibrate your units.
Always handle the NT Head with care. It is built sturdy, however, when you have additional rails connected to it, it is very easy to put a lot of stress to the internal gears when you pull or push at the end of the rails.
Never try to rotate the axes by hand. This will damage the internal worm-gears. Always use the knurl screws at the side of the unit which are connected directly to the stepper motor when you need to rotate the axes.