I don't recall seeing anything on the glue used on fletching. I would suspect that it would be hide glue however. Fish glue was used in the production of composite bows because it remains more flexible than hide glue. In addition its' main use in medieval Europe was in the production of artwork, parchment and paper products. It was one of the common sizes for parchment and was used in the production of illuminated manuscripts as an adhesive for gold leaf and the production of ultramarine (ground lapis lazuli). I have not seen studies of the amount of fish bladders or prepared glue imported to Central and Western Europe in trade studies, but I have seen mention of the expense and difficulty in obtaining top quality material, and several artists "handbooks" give recipies for lesser quality fish glues to be used to cut costs or in case of zero supply. Historically the best fish glue was made of the imported swim bladders of sturgeon from Russia which made it rather expensive. When one considers the fact that most (all?) of the surviving medieval bolts are leftovers discovered abandoned in the corner of an armoury somewhere or preserved as a trophy, it would stand to reason that they would have been made as economically as possible. All of the bolts that I have seen published are of this type and would have been purchased as part of a lot order that may have run into the hundreds of thousands. They have uniformly had wooden vanes, either complete or the remains of them, although I do recall reading somewhere that bolts made for hunting had feather vanes as they flew more accurately and quieter.
If I am recalling correctly (anyone know what source I may have read?) as mentioned, hunting bolts may have had feather vanes, and here it is more probable that the much more expensive fish glue was used (fish glue has a very high initial tack, and like all animal glues, is self clamping). After all, we do have records of hunting arrows being fletched with peacock feather, bound to an ash or poplar shaft with the finest imported silk (red is the most common colour mentioned) and protected with an antifungal/herbicidal/insecticide compound made of verdigris (archaeological speculation as to the purpose) so the use of top of the line materials certainly cannot be discounted.
It is also likely, although it is complete speculation on my part, that rabbit skin glue would be used as it remains one of the most flexible of the animal glues when dry, which would make it less likely to set up a too solid glue line which might be prone to cracking under the stress of being loosed. Until someone can produce a copy of a chemical analysis of the adhesive on an extant bolt, I think this is about as far as we can speculate.