Need advice on how I can dial my single shot [Gaggia Classic E24/Baratza Encore ESP]

Hi everyone, I’ve been using my Gaggia Classic E24 for about half a year now. I use the standard portafilter with the two baskets that came with it. When I use the larger basket with 18g of espresso, the coffee tastes great. As soon as I switch to the smaller basket with 9g (I adjust the grind size), the coffee either flows too quickly or comes out very thin. Do you have any tips on how I can get good single shots? Should I buy a new basket?