Are you taking the measurement with the built-in antenna tuner
enabled? That will reduce your output power somewhat if you
are using an external meter, but the difference doesn't show on
the internal meter.
Also, make sure that your ALC / POWER OUTPUT knob is full
clockwise.
If you don't have enough drive power to activate the ALC, then
your radio may need alignment.
But, no, transistors don't decay over time the way valves do.