My boil kettle is about 56x85cm, so relatively tall and narrow. I get a good rolling boil but no matter how much hops I throw in in the last 5 minutes I can never seem to get any aroma whatsoever. I'm using a very large fine-meshed filter bag to through the hops into. Could the poor hop utilisation be due to the kettle shape and size, or more likely the hop back (bittering is fine)? Or otherwise, any other ideas? Thanks.
What hops are you using? Some hops do not make good aroma hops at all, they are bittering only.
How old are they? The older a hop, the less aroma you'll get from them.
How do you store them? If they're stored without oxygen, in a freezer, they will be fresher when you use them. The less fresh hops are, the less aroma they produce.
How do you ferment? Non food grade plastic buckets could absorb the aroma. If you're using cleaning materials that have perfumes, or you're not rinsing correctly, you may lose aroma.
How long do you ferment? If you just use primary and it's short, the aroma may be over powered by other aromas (such as yeast or malt), or if you have an excessively long fermentation, the aroma will dissipate.
Do you dry hop? If you don't, you may want to, dry hops give the most aroma.
How do you age? The longer you age your beers, the less aroma that will be available from the hops when you serve/drink the beer.
How do you server? From bottles, you may have left them too long, or the bottles may not be sealed well and lose the aroma. From keg, check how you're cleaning it, is the keg leaking gas? That may lose aroma.
In short: There's no clear reason why without a lot more information, a full breakdown of your process and your recipe, you may just not be adding much at all!
If you take a couple of hops and rub them between your fingers what aromas do you get?
If you are getting punchy vibrant aromas then these should be in your beer, may be split your aroma additions 50/50 between a 15 min addition and your 5 min addition.
If you are getting very little at this point then get new hops for aroma additions, and check these in the same way.
You can add 100g of aromaless hops at 5 min and achieve very little where as 20g of pungent fresh citra should be adding a noticable citrus punch to a 5gal/23l batch.