An interesting observation:
I too have had trouble with Boltardy beetroot seed. A packet bought this year from Plants of Distinction sown in different composts at various intervals has produced a grand total of 6 seeds germinated.
At a local garden club meeting I won a couple of packets of beetroot seed (different varieties) in the raffle. The sow by dates were 2017 and 2012!. I was not expecting much so sowed them thickly in modules. After less than a week they are both showing healthy germination rates.
I am beginning to wonder if the boltardy problem is more than just a slight age problem?