As you are starting to experience, there are a lot of variables in coffee making.
What beans are you using?
Do you know when they were roasted?
Are you letting the Silvia warm up with the portafilter(without coffee) in place? Most machines, and certainly the Silvia, benefit from a 30 minute warmup before you start pulling shots.
Are you flushing some water through the group prior to doing your shot?
The flow rate sounds OK, but how does the coffee look? Is it rich and reddish brown or pale and light brown to yellow?
So many questions.
0-15, your serve!