If you haven't got a male on the premises it can be really hard to detect any females in heat. My goats are sponged in early October (so I can finish kiddings before lambing starts), and no injections are necessary at that time. (But I have not seen mine in heat before October either). In 2009 it all worked very well and everyone was very noisy about two days after sponges came out. Billy rag worked well too.
However last year my BT did not show any signs of being in season after sponge came out, and I had thought given the noise the previous year I will be able to tell and didn't have a billy rag... I managed to get someone to post one really quickly, goat wagged her tail on smelling rag (This was day 4 after!) and she was whisked up the road in no time. Mating was successful, but I think only just!
Really, I would now wait until September, the vet can insert the sponges, and you should have kids early on in January. Also if it doesn't work you will have enough time to try other things to get them into season. maybe having a boy on site would be an option?