The OS has a lot of drag in its bottom end,especially when dry and cold---it has a heavy crankshaft and a wider-than-usual rear bearing cf other 140-160 2-strokes.
If you give it a good big manual prime straight into the open carb throat,and flick it over by hand a few times,before applying the starter for the first time on the day, it might free up enough for the starter to whirr it satisfactorily.