There are a number of factors that may be at play here.
Board width will certainly help with early planing, so a wider board like the F-Type 158 should get you a knot or 2 in the early planing realm.
Skills will be a big factor.
If you can "ooch" a board onto a semi plane and then pump it onto a full plane, you can plane significantly more.
Sail Type may be a big factor as well.
You have the 8.0 m2 Retro which is one of the best light wind "powerplants", but you will need to rig it as full as possible and use an adjustable outhaul to extract all of it's marginal wind performance.
Wind Quality is another huge factor here.
If you have a nice steady 10 knots, you will have a little trouble getting going, but once on plane the wider F-Type (or a formula board/GO board/large Isonic) will go into "apparent wind" mode and you should remain powered up pretty easily.
If you have fluky, gusty winds with alot of holes, you'll be quite frustrated as some of the time you will be powered, sometimes just slogging along with no way to get planing.
So, if you have a steady or "gusting above" 10 knots of wind, and a 90 cm or wider board with a 55 cm or longer fin, you should be able to plane nearly all the time with your 8.0 m2 Retro once you figure out some of the technique required to get onto plane really early.
Hope this helps,