Big Red lens needs cleaning maybe?

I have been cutting quite a bit of 6mm baltic birch. When I started I’d use 100% power and 3.7% speed and .23" Z. That would cut thru 99% of the path but left a few spots not quite cut thru. Today it cut less than 50% thru. Even 2.5% speed did not cut thru more than 95% on my last piece. Maybe my sheet just had more glue than typical but it seemed like the laser intensity has dropped the last few times. Maybe the lens is dirty? Other thoughts?