Leaking Sliding Window During Heavy Downpour

[mummychua 4]

I have moved in my place for 3mths. On the 1st night i'm there the my sliding window leak due to heavy downpour. Called my contractor and he got someone to cut a bigger hole on the holder to allow the water to drain quickly. I haven't got any problem until a heavy downpour during last weekend.

i will like to know if this is an unavoidable problem with sliding window or is this a problem with faulty installation with my contractor? is there any quick fix for this?

my contractor has sort of disappear without finishing up all the touch up and defects and i'm not too keen to call him back as there are still pending payment but not because I didnt want to pay him but he didnt even give me the final invoice after 3 long mths.