I've glued 2 lap size quilt sandwiches last week and the one thing I did differently was put little random glue 'dots' on the (batting) about every 4-5 inches instead of 'lines' or 'swirls' of glue. You really don't need a lot of glue to be effective. I'm still looking for half a needle that broke off while I was FMQ when I think I hit a fairly large spot of glue. Remember it dries and gets harder!
What works well for me:
1) lay the batting down on the floor (I don't tape or anything).
2) lay the backing on top of the batting and fold half the backing back on itself.
3) working from the middle, I put small dots of glue on the BATTING over an 18 inch wide section, smooth the backing on that section and iron that section.
4) I then do the next 1/4 section working towards the edge. (repeat until the whole thing is glued down. Make sure everything is flat. (BONUS: I didn't even have to iron my backing ahead of time because I didn't need to tape it taut to the floor!)
5) Flip the whole thing over so the backing is now on the floor with the batting right side up.
6) lay the top on top of the batting and repeat. It's just so slick!