I had this same problem with Auto Windscreens back in February ... and it was only resolved last month! After ALOT of phonecalls and a couple of complaints! Think they thought I didn't know what I was talking about as I was a girl
Like you say the wipers would work fine to start with but after a week or so they would gradually get worse and worse to the point where they wouldn't work on auto at all.
My problem was with the gel pad where the sensor is on the windscreen. If you look at yours ... is it like a milky colour? If so then you probably have the same problem as me. For the sensor to work the gel pad needs to be clear i.e. pushed firmly up against the windscreen.
Auto Windscreens replaced my screen THREE times trying to rectify the problem (after trying to fob me off I must add!). Where Auto Windscreens were using 'copies' of the original Renault glass the brackets that hold the sensor bit were bending (crap quality) which is why the gel pad looked milky coloured after a week or so after gradually coming away from the screen and thus the sensor was not working properly. If I pushed my hand hard up behind the rear view mirror making the gel pad appear clear again from the outside, the wipers worked fine which is how I knew it wasn't a faulty sensor ... but trying to get them to believe me was near on impossible!
In the end I INSISTED I had a genuine Renault glass and hey presto the problem went away!
Hope you get it sorted ... windscreen companies are a nightmare!