How can I fix a loose ferrule?

Allowing a ferrule on a stock to be loose or cracked is just asking for trouble. The purpose of a ferrule is to provide support for the female joint of instrument so that it won't crack when you force the drone, chanter, blowstick or tuning slide into it. Without the support of a solid ferrule, the receiver is more likely to crack.

It's not uncommon for ferrules to loosen up on new bagpipes (as the wood dries/cures) or to come loose in the winter (when the low humidity causes the wood to shrink). Loose ferrules can be re-glued with epoxy glues. (Lately, I've used "Gorilla" Glue with great success.  Be aware that this glue swells to several times the volume you use, so there is a high potential for making a mess.  However, it requires no mixing and is somewhat flexible.) If the space between the wood and the ferrule is particularly large, wrap a few windings of non-waxed hemp to fill the space before gluing. This allows for centering the ferrule over the wood.

If you've lost the ferrule or have a cracked ferrule, they can be replaced by most bag piping manufacturers or shops. In the meantime, your bagpipe is at risk of being cracked. Although it won't look great, an automotive hose clamp will provide the same purpose until you can get a permanent repair.