It could be that she didn't take, or could be that she absorbed the foetus early on and came back into season? Although most are fertile, not all ewes are and some ewes just won't stand properly for the ram, which is worth bearing in mind, particularly as I think your ewes are young.
One of my ewes didn't get in lamb until she was 3, she didn't take the first year and I gave her another chance, she didn't take again the following year and I meant to cull her, but never got round to it, the following year she did take (lucky for her!)