Dermestidae beetles are more then likely what's eating your mounts. They are after the meat still in your mount and hide that is not tanned. There are two main ways to tan your hides. One is a commercial operation that uses harmful acids that are very expensive to maintain and dispose of. This costs the most money for the taxidermist, so some will use the "pickling" method where they do it themselves, or go to a non commercial operation where they pickle the hides. Basically they just dry them out and then soak them in a solution that makes the pliable to mount. These are the hides that the bugs love to eat. The hair will slip, and they will eat around all the edges of the cape.
My taxidermist used Golden State Tannery in California, even though he is in East Texas. He feels that they are worth the extra expense and time to deal with shipping them there and back, and I agree. I have no problems at all paying a little more for the best tanning of my hides possible.
With some horned animals, it is impossible to get the horns off the bone to totally clean all the meat that is between them. Boiling and leaving them in the bug boxes will get a lot of it, but never 100%. This is very common with some of the African animals. The shape of their horns will never allow the taxidermist to get them off. After all the cleaning is done, they will pack the inside of the horns with borax. When done properly, there should be no problems with bugs.
This is a big part of what separates a good taxidermist from an amateur. It is also very common in mounts done overseas and a good reason to pay the extra to get them done here.
Bug bombing them works, and there are several powders that you can dust your hides with that help. This is a never ending process that needs to be done every couple of years, or even yearly now that it's already started. I'm not familiar with the powders other then they exist and I know of them. I don't have any bug issues, I've just seen the damage being repaired and talked to my taxidermist about what he was doing to fix it.