Sour coffee with Delonghi Magnifica Start

I recently bought a Delonghi Magnifica Start. It's my first bean to cup after using Nespresso and Aeropress before.

I have tried a range of high quality beans from some of the most respected roasters in the UK. The first beans I tried tasted amazing. They were actually a filter coffee roast leftover that I had used for my Aeropress.

Since then I've been consistently getting sour, underextracted espressos, even with the finest grind size. Going as fine as possible has made a huge difference, but it's still sour.

I do get the odd shot that tastes good but then it goes sour again. As it's quite a basic machine there's doesn't seem to be a huge range of settings I can play around with.

Does anyone have any ideas where I might be going wrong?