I have not worked with FWs decal sheets but, if they are like the ones in the GW kits at all, they are probably too thick.
Here is how I approach decals...though this may be more than you're willing to do.
1. Apply a coat of gloss to the area where the decal will go and let it dry. Decals adhere to smooth, shiny surfaces better than rough porous ones.
2. Cut the decal as close to the art as possible. Try and not let there be much of the clear "film" as possible.
3. Soak the decal until it slides off the paper and apply it with water. Using a brush or q-tip get the water slide is the proper place and then remove all the water you can. I do this by rolling a q-tip over the decal itself...both removing water and pushing out air bubbles.
4. When the decal is located properly and dry then apply one coat of micro set and let it dry. If you're going to ding the decal up, now would be the time.
5. Apply a coat of micro sol and use your brush, from the center of the decal out, to push down the edges...let dry.
6. See step 5....do step five several times (when I did some Ultramarines the GW decals took about 4-5 step 5s). Each time brushing from the center of the decal and working out.
7. When you are happy with Step 5 give it another coat of microset and let dry.
8. Give the area and decal two coats of gloss, letting it dry between coats.
9. When the model is finished you can matte or dull coat.
The end result should leave you with a decal that is sealed, smooth, an part of model.
Again, it is a length process but it work. Patience is key.
