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trace the shunt tube cathode, it goes to the 1k. You need to make sure its not drifted, should be a 5%. the 1k needs to be accurate to determine the current in the shunt, as its the voltage drop accross that. I generally just hookup a multimeter on both sides, take a resistance reading (before powering up the set) should be 1k, then switch to voltage and power up, you should see .5 to 1.5 depending on how the shunt in working (tapping off excessive voltage on dark scenes, the v drop will be the highest showing current flowing thru the 6bk4, bright scene the voltage drop will be much less, around 200-500mv. The idea here is to determine if the lack of HV is due to a the HOT/Fly or the shunt. if the shunt current is low in a dark scene then it mean the HOT/FLY is not puttting out enough HV to start with (so you check that there) if the shunt is clamping down hard even in bright scenes, then the HOT/FLY is prob ok and the shunt circuit needs attention.
You need to narrow down the problem, or you could get lucky and just keep replacing parts hoping to find the problem.
Obviously the 1st thing to check when you have HV issues is the big 4
HOT/HV rec/damper/shunt.
you can just sub those with know working tubes and if that fixes the problem start replacing one tube at a time with the orig. The problem with this approach is you may or may not have good tubes to sub with, SO you need to think the problem thru and apply what the test that have been described.
Besides all this you really should setup the HV that means the the the eff coil and the HV while checkng the HOT and Shunt currents (as has been described several times already), skipping this (just replacing tubes and adj the HV) can result in unnessary stress to a flyback that can result in damage of a part that can be VERY hard to find.
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