Hey, Welcome to CS!:)
1. 15mins isnt enough for the silvia to get up to temp, it should take more like 30mins.
2. Keep playing with the tamp pressure and dose and grind. Keep playing with those varibles. normally we would say 25ml in 30sec would be quite reasonable, so just try and aim for that to start with. if its black and dark with oil it is over extracted (comes out to slow), and a very light crema colour that dissapears very quickly is probably under extracted (comes out too fast)
This probably aint the best answer but someone else will soon jump in and make it more clear :)