Chances are if you are using a standard sluice that you are missing a lot of the fine gold to begin with. I am the developer of a new type of sluice that excels at recovery of fine gold (and large gold too!). I live in a gold-rich area of Arizona that has a lot of very fine gold, and that is what prompted me to develop this sluice. A completed run through the sluice yields only a small amount of material (less than a cup, mostly black sands and gold). A typical cleanup takes me about 10 minutes or so. After large gold removal, I keep the black sands and reprocess them through the sluice when I have a large quantity, thereby concentrating the super-fine gold to a point that it is more easily separated and recovered.
I have processed concentrates that have been put through spiral wheels and blue bowls and have recovered gold that was missed by them. I have processed nearly pure magnetite and recovered flour gold from that as well. Although it is not a final recovery system, it will greatly reduce the quantity of unwanted material to a more manageable amount. It can be used as a primary sluice or a cleanup sluice.
This sluice works by creating vertically oriented vortices, the top 'bed' of which is constantly in motion. When something heavy passes across the top of it, it drops away through the material on top into the low velocity area below. Imagine a bucket full of ball bearings being vibrated around and dropping a coin in it. It's going to the bottom. Material should be classified to 1/4", however I have used it with raw material without much problem. The horizontal slots across the sluice are 'mixing wells' that accelerate the separation of the light and heavy material so that it flushes out quickly. They will grab a nugget or large flake as well. If this seems like it might fit your needs, PM me.