I know this is not the answer to the problem but, I find i get a better response from clients when I am in constant communication with them.
If they start to 'drag' hopefully by then I have established open lines of communication and asking then what the fuck is going on usually gets a response good enough to let me adjust my time frame and balance my work accordingly.
I know its not the answer you are looking for but if this is not usual practice it is worth a try.