Have you tried using a line weight heavier? I have an 8wt rod I built for steelhead and silver salmon and went on my Alaska trip with a 9wt WF floating line and found that I was still under lined and ended up with a 10wt line in order to properly load the rod.
Go down to your local fly shop or sporting goods store and ask if they have some different weight lines on reels that you can try casting with your rod. If they're anything like my local shop (Jimmy's All Season Anglers, Id Falls, ID) then they would be more than happy to help solve your problem. Don't just take the line weight off of the box as the one that will work.
Anyway, hope this helps.
Tight Lines,
Kelly.
"There will be days when the fishing is better than one's most optimistic forecast, others when it is far worse. Either is a gain over just staying home."
Roderick Haig-Brown, "Fisherman's Spring"