First I would try to find out if there was a larger problem. This might not be about your material, But to be sure about that we have to isolate the material as a variable.
I would try to run a gift of good measure on proof grade material and see if it behaves as expected. There are a lot of possibilities as to why you may experience a change in your power level, a deteriorating tube is thankfully near the bottom of the list. It’s much more likely that there’s some portion of your cleaning procedure that wasn’t done correctly, such as missing a critical step or inserting your mirror and upside down.
You can narrow the possibilities down by running a gift of good measure with known proof grade materials. If that doesn’t work, then there’s something else going on. It might be the tube, but it’s more likely something simpler.
Even if you’ve had your laser for a long time it’s not a bad idea to go through the documentation for cleaning and make sure that you didn’t miss a step. I know there’s been many times where I have been moving quickly and skipped something.
You can find it here if you don’t have the link handy.
https://support.glowforge.com/hc/en-us/sections/11792691970075-Cleaning
If you go through the cleaning procedure and you’re set up correctly, and you’re still having trouble with the gift of good measure, the next step is to take a look at the standard troubleshooting for cut issues which you can find in number five on this list:
Again, apologies if this is all too rudimentary, but like many of us you’ve had your machine for a long time and it’s easy to skip a step when you think that you have seen it all. I still refer to these lists if I have some sort of an issue, it just helps keep me on track to be thorough.