faulty aircon cleaning- need advice

I had our split aircons cleaned about 4 months ago. Since that time, one of the aircons leaks water once every every 6 weeks- I shut it off and call the cleaning company. The cleaning company sent some guys twice to fix the problem. First time, said the drain pipe was blocked. Second time, said that the pipe hadn't been connected properly. Now, it's leaking again. Never had this problem in the 5 years I've lived in this flat- that is, until this company cleaned the aircon. Does anyone have any idea what the real problem could be? It's annoying that I'm trusting them to know what they are doing, yet they can't seem to fix the problem & I can't doublecheck their workmanship. Any suggestions welcome!