Each year, you should apply a good quality wax like Butcher's wax on the limbs in the direction of the scales. If the finish is failing over them, which you may not be able to see, this wax gets in, permeates and seals them up.
If getting older, you can use 600 grit sandpaper and give a few swipes in the direction of the grain. Vacuum, and spray with a high quality finish like Varathane. Give a good week to cure, give another wipe with the 600 grit, vacuum and spray again.
A couple of weeks to cure and then wax.
If the skins are lifting anywhere, you can apply small pieces of paper towel, soaked in warm water over the bad areas to rehydrate. Once pliable, use super glue and toothpicks to get the glue under the damp skin and reapply with pressure. Let dry a good day and finish like above.
Depending on what type of finish is on them, all you may have to do is apply some mink oil type paste, like pitch blend in the direction of the grain. Let sit and wipe with the grain several times. If this works, there isn't much or any finish over them and you need to do this every couple of months. Which will keep them in great shape.
You can touch up worn spots with a fine tipped sharpie marker, no one will know where you have done this. They come in browns and black. Then wax or finish over this.
I have bows in service that have been skinned by me for about 20 years and all in between. Take care of them and they will last many, many years.