There are a few things it could be but let's rule out the simple one's first
At the exit end of the tube is a focussing lens, ensure that is clean first. If there is any muck on that it will cause the beam to diverge and spread reducing the effective power (90% of a beams power is in the outside 10% of the actual beam).
At the opposite end of the tube is a mirror, check for any burning or signs of heat in the area around that and any signs of any air bubbles in that end of the tube (the small glass chamber right at the very end is the usual culprit).
Alignment you seem to have covered but always worth a check at the final mirror as mine went out yesterday on a machine only 5 days old. It can happen when a machine "settles" or gets bumped or moved.
Water temperature, heat reduces power by a HUGE amount, what temp is your machine running at? (A fish tank thermometer in the water tub is great) anything over 25 degrees and your losing efficiency of the tube hence cutting power (lower temperature water = lower resistance in the tube = more power).
Any bangs, cracks or pops? When a tube dies it usually makes quite a noise (I don't think we are there yet)
Power supply is very unlikely, yes they do die but it tends to be more of "It works" or "It doesn't work" with PSU's,there doesn't seem to be a lot of middle ground. If you do suspect PSU please please DON'T
do anything with it, the amps are very low but the voltage even on a 40 watt tube is in the 22KVa range and WILL
kill you if you get a blast across your chest.The spark will jump 2 to 4 inches as well across free airspace too....
Connection wise , I use crocodile clips and they provide a good join, the trodes are titanium allow so don't solder very well but always worth looking to see if they are arcing or burned. (If you have had previous arcing problems that can damage a tube, even one with less than 10 minutes life on it)
At the minute I'm leaning towards
1:Dirty tube lens
2:Leaking connection at the electrodes
3:Previous tube damage from arcing
4:Bubbles having damaged the tube mirror
But no necessarily in that order.
Let us know what you find Phil and if no luck I'll see what else I can come up with.
If you get really stuck with something I'm on 07709066493 pretty much 24/7