Could be a cracked solder joint on the tube socket. These are PCB mounted sockets so that is not uncommon. If a known good tube "isn't lighting up at all" it means the heater is not getting voltage. Assuming this is 12AX7 the heater pins are 4 and 5 with pin 9 being a center tap of the heater. It should have 6 volts AC or DC between pins 4 and 9 and also between 5 and 9. Sometimes pin 9 is grounded but sometimes it floats and they put 12 volts AC or DC across 4 and 5.