I don't think the time frame is relevant.
But i have had times where i've tried to give rep to a few different people and it kept telling me that as well. I assumed that you have to spread it out more than you think.
I don't know how much rep you give out, but under 'My Chat Profile' it shows the last 7 people you repped. Perhaps check there to ensure you haven't repped the person before. Mine have four reps going back to Dec 2013 haha.
Otherwise that may require a mod response.