If the coffee and grind haven't changed but you are getting less extraction then you are losing pressure somewhere in the system and it is likely to be from the steam wand.
The most common issue with this is a build-up of calcification on the tip of the steam pin.  This can often be fixed by tightening the steam knob several times (by hand) as hard as you can which will break it up.
If that doesn't work then service would certainly get it back working well again.