if I have nine healthy ewes and 13 healthy lambs at the end of April, I'll be well pleased. 
Quite! With a 300-head-ish commercial flock, we are happy if every ewe rears one good lamb - good singles grow better and are away sooner, when prices are still high, and usually get away without any treatments, or being weaned, or any cake, etc.
And a ewe that has a single this year will probaly sail through rearing twins next...

Bit disapponted with two of the singles in the ewes as they both had twins last year (tried it, didn't like it?). Maybe didn't get the flushing right this year.
Ach, could be so many things. Flushing is just about ovulating eggs. They have to be fertilised, implanted, the embryos have to be ok, grow ok... the ewe's nutrition has to be right, a shock can cause one to be lost ...
If the singles-this-year-twins-last girls are 2nd-croppers, I'd say they're being sensible! 3 lambs in the first 2 crops is pacing themselves sensibly!
