I used a similar lever machinbe for a few years...but I dont consider myself an expert ;)
I think it has alot to do with lack of temperature stability. The grouphead gets too hot after the second shot. I always found (as many others seem to have based on my reading) that the first shot was so so, the second the best and the third often burnt due to the group being too hot.
Some advocate using a tea towel soaked in ice cold water to cool down the group head. I used to turn it off and switch back on when having another cup, but its really not suitable for making multiple cups in a row. I also found that it was very unforgiving when it cakme to variations in grind and dose...you need to be spot on!
In the end while it wa a great way to learn about making espresso getting a HX machine was a big step forward for me.