Im not familiar with the exact machine, but.....
Are you using a double or single basket?
How much liquid in milliliters are you extracting over 30 seconds (approx. 5 seconds of "preinfusion" while the pump goes but no coffee comes and + 25 seconds of flow)?
What is "wrong" with the espresso (you should be able to identify this even if you dont really like to drink it)? Does it taste sour (towards the front/tip of the tongue) or does it taste bitter (towards the back and sides of the tongue)? or is it a combination of both?
A good shot should sort of balance nicely on the palate.
What does the coffee look and feel like in the basket after you pulled the shot? Are there any channels or wormholes? If you push on it with your finger does it feel firm or does it feel mushy and collapse under the pressure?
Are you preheating your machine for long enough? Letting it warm up and running a few blank/empty shots to warm everything up?
The answers to the above might provide some insight into whats going wrong