Being unable to bring tubes up to recommended the bias range often is due to tube aging. If those are old tubes it may just be time to change them.
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Newbee is correct. I would also check the actual bias supply voltage. It's possible that it's low to begin with in which case either resistors or a resistor have changed value. It's possible you have a bad filter capacitor causing the lower than spec. bias supply voltage, or you have a rectifier diode or diodes that are leaky and need to be replaced. Happy hunting. G