My observation of Osteospermum is that it doesn't bloom well in the heat of summer under good conditions, more of a cool weather plant that goes green in summer and will bloom again towards fall. However, I haven't really messed with them for a few years, some of the newer ones may be better in the heat and longer days. Dahlias, as you already know, are for the most part short-day plants, although it varies and some will bloom better in the summer than others.

About all I can suggest is using a fertilizer with moderate N and higher in P and K to help promote flowers, there are a lot of these "bloom booster" type fertilizers on the market.