I agree with Paula Suárez-López. You should try to make your days longer, if possible. We have used the WS ecotype extensively, and it always flowered about the same time as Columbia and RLD. When we were using WS, we routinely used 24 hr day length.

...thanks a lot for the comments! i will first try the GA treatment because we don´t have any 24h light chamber!

I can figure out of the name (there is no exact data in Genebank), that ecotype Wassilewskija is from Vasilievski Island, St. Petersburg, Russia - city of white nights at summer. So, use addition light at dark time and lower temperature.