I know i am late . But for anyone else who in future faces this issue.
I read a lot of other posts that recommended the use of export parameter instead of TABLES parameter.
Hence,I tried retrieving a table from the export parameter of function module in the web service, but the response was empty.
eg. <LtFlight/>
The FM was working perfectly but returning an empty reponse in the service.
I then removed the export parameter and instead added it in TABLES .
eg. lt_flight like str_flight (where str_flight is a structure)
And this worked.
The web service was now able to get all the data as required. 
U can always use either EXPORT / TABLES . Sometimes the response types maybe not be recognized ( like i received Unsupported xstream error HTTP code :500 ) and that is why the response is empty.
Hope this helps.