You need to understand the sequence to make the best use , rams need to kept out of sight, sound and importantly smell from the ewes for 6wks or so before you wish to mate , then a max of 14 days before mating either put the ram in the next field or a teaser in with the ewes , this will stimulate the ewes to cycle but the first cycle may be silent so no mating will occur , then the second cycle will happen 17 days after ram stimulation and mating will occur. From your post this is what seems to be happening at the moment and why the 2nd and 3rd wks produce the most lambs