First thing I would do is test them. Most decal sheets have unused decals. For example, there is probably an identification of the kit, and maybe the mfgr's logo. For arcane reasons the decal maker usually makes even that stuff as an actual decal. Cut out one of those areas, and test it by trying to use it on some scrap.
If the decal shatters when you dip it in water, this can be fixed by painting the whole sheet with a decal film, such as that from Micro Scale. That, however, turns the sheet into one big decal, so you have to trim each decal with a knife or scissor to cut it away from the sheet.
Another fault is that the adhesive may be gone. While there are some fixes for this problem, they aren't easy, and if that is the problem I'd go for the aftermarket decal. Some vendors such as Squadron, Roll Models, Sprue Bros, etc, have pretty good search engines at their site and you can narrow down the search terms pretty well.