Since the 19th August patch, my client will not show any Dual Heavy Cannon effects, of any kind, from any player or entity; not even myself. I noticed all my cannon fire vanished on the 19th, and duly updated my drivers, this brought back standard cannon and turret fire, but not Dual Heavies.
I run a Windows 7 (64bit) machine, with an nVidia GTX 460SE (1GB), 6GB RAM, 2.3GHz quadcore processor.
I seem to have been very lucky not to get many of the issues experienced by my fellow GTX owners, and I hope Cryptic will do something about this soon.
Just thought I'd make an update on this, after re-installing the game (twice) I am still experiencing the issue. I have this problem on all 3 servers (Holodeck, Redshirt, Tribble). And yes I have made a bug report on each server.