On the many Noblesse variants the cap pushes on and is held by friction alone. If it comes to pull off too easily with time, that fit has worn loose, which is ordinary wear. The grip cannot be adjusted, and with these models a repair is as a rule not worth it, because the labour and the hunt for spare parts come to more than the pen itself is worth.
The most practical course is to pick up a second example cheaply as a donor and take its cap. Used Noblesse pens come up for sale regularly, often at prices well below the cost of a workshop repair. It is important to look for the same variant, as diameters, fittings and finishes differ from one version to another and a cap from elsewhere will otherwise sit just as badly.
Until a replacement is found, the pen should not be carried loose in a pocket: a slack cap comes off unnoticed, and the unprotected nib is easily damaged in the process.