Velux are not sold as maintenance free for ten years, you need to carry out a bit of cleaning and lubricating etc.
That said your windows shouldn’t be leaking but you will have to try to find out exactly where the water is entering the window system. If you have access to the windows try a small water test (with a hose pipe) but don’t overdo the water as you’re trying to find a possible leak not cause one! Start at the bottom of the window and slowly work your way up to the top of the window. Have someone on the inside to observe the window and if or when the water penetrates the system, photo it and note down where on the window you noticed it. If the leak due to the flashings installation then that’s not a product fault, if the gasket is the source then it’s fairly easy to replace the gaskets, if the joints on the window are leaking then you still have a couple of options before you need to start ripping things apart.
PS once again, please don’t use too much water when you carry out a test, different windows are designed for different applications, cwct website has some useful info.