Sounds like you are doing most things right. I'll make two suggestions and then let more experienced people jump in. Firstly, you can experiment with change in dose. You can either up dose and grind coarser or down dose and grind finer and see how your extractions progress. If your shots are tasting under extracted then perhaps try down dosing and grinding finer. The other thing is the temperature of your brew water. What have you set the PID for? Perhaps you could increase temperature and see if this improves extractions.