Sage Barista Pro steam wand issue

I’ve had my SBP for about 7 months and I’ve always had an issue with the steam knob.

The first jug of milk I steam is usually fine, but when I steam a second jug of milk for the second cup of coffee, the steam knob becomes extremely stiff and although it’s turned all the way to the steam setting, it will only stay on the steam mode if I hold it in that position. What also happens is that a lot of steam comes from the drip tray and the steam wand then seems less powerful. When I let go of the knob, the steam will stop and the machine won’t acknowledge that the steam knob is turned all the way to the steam position.

I’ve completely cleaned, descaled and flushed the machine but this issue is still ongoing - any suggestions of how to fix this?