Scopes tubes aren't forever, and the 1st tip is a low bright before you need to buy a new tube,
Bright in an analog scope is function of several things
1) High voltage, power supply ( most problems come from here ), or disconnected cables
2) Filament power supply, very few problems comes from here, but if your tube is on near end of life, you can get some extra time adding 10 20% tension to the filament and pray that they did open, But you are already in a situation that the tube is end of life, the maximum you will get is a total blank and need to a new tube, It's the last try before buying a new tube
3) Grades control system, Several trim pots are involved on this part for bright, focus and retrace blank, this can be another point of low bright problems.