I am an FG fan, but the knot is very easy to screw up. Also, it doesn't like an aggressive twitching retrieve. Same for the Alberto. My suggestions are: 1. More wraps than normal as recommended above. 2. You have to pull it VERY tight to set it. What the knot actually does is deform the leader to form mechanical interlocks in the leader for the braid wraps, but if the braid comes loose, the knot is gone. So the next suggestion is 3. Concentrate on very tight, multiple, half hitches to finish it, then apply super glue or UV curing epoxy to just the half hitches.
If you take one of your FG knots apart and see no permanent distortion of the leader, you are not tightening it enough. If it slips as you pull it tight, you are not using enough weaves. If the knot continues to slip as you pull, start over with more weaves-this knot is not salvageable. The FG is also different with lighter lines and leaders than with heavy ones for which it was developed. For lighter lines and leaders you really have to use a lot of wraps, and very tight, multiple, glued half hitches.
I know there are those who don't seem to need to do all these things, but I wonder if they really use the knot with aggressive twitches. I cannot get away without doing them.
The FG done right is a dream, very strong, very long lasting, very clean and quiet through the guides.