With large fins, sometimes "pumping the fin" is very effective when you are not quite able to get going, or can pump up onto a plane, but the plane dies off as soon as you stop pumping.
The idea here is to give several quick hard "pushes" on the fin with your back foot (either in the strap, or just in front of the back strap).
Sometimes this seems to "loosen" the board up a little and can be the difference between progressing onto a full plane, or dropping back into "slog" mode.
It also works if the wind dies down a little and you are starting to lose the plane.
2 or 3 quick pumps on the fin and heading a little off the wind (if you are higher than a close reach) can keep you going until the wind comes back.
Hope this helps,