About a million threads on this! Known issue and it'll be a clogged jet nozzle, or pipe leaking/disconnected and possibly damaging your rear wiper motor. Do a search please and read the many related threads.
it will more than likely be the jet is blocked due to amount of salt being deposited on our roads. Only problem is the more times you operate it the more chance that the build up of pressure will find the next weekest link to pour out from. Rear wiper motor, connector fills boot lock with water alarm goes of at 3:00am strangely enough. Best advice dont operate the washer until you can confirm the water isn escaping anywhere else
I had this recently and it needed a new washer jet. It cost £80 all in, including parts and labour at Audi main dealer. Very reasonable price I thought. Hope you get it fixed easily mate.