Other than obvious problems with cloning you might encounter like a dirty old system with bacteria I think its important to pay attention to the environment the cloner is in, just like a plant. Temps and humidity are important factors. I find I cant clone in the garage year round successfully because it gets to cold in the winter. Same could be said in your house, depending on the person, temps can range quite a bit throughout a year if you arent one to run the AC or heater much.
If things used to go so well and now they aren't, something has obviously changed and it may be something very minute you're overlooking.