mand what is the temp of your room , its doesnt fluxuate does it as you know our climate is a head ache and the only time i have had droping at the tips is when its been cold outside i will post your pics to a freind who is a botanist to see if we can decidie what it is try take a pic of the wirst affected plant you have or the worst affected leave and we will get to the bottom of it just for you mate
love you best weee old mate
scobbie