as with all crafting, if you want to make +1 items, your best chances are on darks day. but be warned your total number of successes is going to be much lower. but more of htem will be +1 (and there will always be an element of randomness no matter what)
I'm Lvl 28 woodworking and get 4/11 +1 boomerangs so... i think bout lvl 18 would be 1/12 +1 boomerangs. Just think that the higher lvl you are the better chance.
Do you happen to know what day you were crafting on? I tried this at 27 skill and got zero on a Earthsday (weak to Wind, Wind powers increased) Perhaps I'd have better luck on an Iceday or Darksday?